Читать книгу Media Blasting & Metal Preparation - Matt Joseph - Страница 10
ОглавлениеBASIC METAL CLEANING CONSIDERATIONS
Whatever great project plans you have, if the metal on which they are based is corroded or otherwise contaminated, your plans will founder sooner rather than later. Although some factors in metal projects cannot be controlled, the surface condition of metal is largely within the range of what you can make go your way for most purposes.
I am not talking about “laboratory or metallurgically clean” here. I am describing the level of cleanliness necessary for successful painting, welding, brazing, soldering, plating, and more. If you think that there must be exceptions to the necessity for clean metal as the basis for some projects and purposes, you are probably right. Cast-iron boat anchors come readily to mind.
Is achieving clean metal easy to do? Yes and no. It depends on your facilities, your skills, your judgment, and the size and nature of what you are trying to clean. Let’s survey some of the possibilities.
The first consideration is the size of your object. Obviously, cleaning the metal in a piece of jewelry is very different from cleaning an auto body, which is very different from cleaning an airplane or ocean liner. In this book I stick with the jewelry-to-car size range and leave the likes of ocean liners, airplanes, and boxcars to the specialists in those fields.
This 1930s fender provides a good example of the enemy: dirty, rusted metal. A band of metal with some paint on it is visible through the middle of the photo, with superficial rust coming through the paint. The top and bottom of the photo show flaking and deep, pitting rust.
The size of an item often determines what cleaning processes can and cannot be used. For example, immersion in chemical cleaners, use of electrolytic “dip tank” processes, or car factory “pickling baths” and “e-coat” painting are practical up to the automobile end of the size range. However, ultrasonic cleaning, another liquid process, is limited to smaller parts because the cost of the equipment for this process increases radically with tank size.
Small parts such as this sheet-metal cover are usually much easier to clean than larger parts. Abrasive blasting, wire brushing, and chemical treatment and dipping are among the obvious ways to deal with them.
With medium-size items you may have fewer cleaning options than with very small items. After you remove grease and grime with solvent (to avoid contaminating the equipment during the next step), a trusty stationary electric wire brush wheel gives you a good start for removing rust and paint.
Large parts and panels such as this 1940s hood offer even fewer cleaning options. Dipping in chemicals is usually impractical. Even with handheld power equipment, wire brushing tends to be too slow and less than fully effective.
Disc sanding is an obvious choice for cleaning this hood. It’s reasonably fast and, if done carefully, avoids damage to the metal. This is how body panels were traditionally stripped of paint and surface rust.
Abrasive blasting with the appropriate media is another way to clean large panels, such as this hood. A lot of air and a well-isolated location are needed, too, along with considerable skill to avoid warping the sheet metal.
Small, delicate items such as this hood shutter thermostat must be handled very carefully to avoid damage. Ultrasonic cleaning works well for this part, but ultrasonic tank systems that can hold larger parts are very expensive.
Some cleaning jobs benefit greatly from automation. The cylinder heads shown here were oven baked for 20 minutes at 400 degrees F, bombarded with steel shot (that was generated airlessly), tumbled in the device shown here to remove the shot, and then jet washed to remove any remaining contamination. The system efficiency is outstanding.
Metals corrode in different ways. A badly corroded aluminum alloy step plate is shown on the left. At center is a piece of copper that is polished on its left side and in various stages of corrosion on its right side. The steel bumper cover on the right exhibits common red rust.
No doubt about it. This bronze emblem is very delicate. It needs special attention to get its surface down to clean metal so that its recesses can be painted and its raised sections polished. Soda blasting, shown here, is ideal to accomplish an initial, gentle cleaning.
Here’s a metal-cleaning nightmare. Nothing delicate about it! This incredibly rusted cap screw should be scrapped and replaced. As rugged as it was, no amount of cleaning can return it to structural integrity. Sometimes cleaning isn’t enough. It is important to know when that’s the case.
Quantity is also an issue in choosing a cleaning process. I once had a contract to clean and refinish several thousand 1-inch-diameter upholstery tacks that had cast, embossed, brass heads. I determined experimentally that I could clean and refinish these items with motor-driven wire brush wheels, composition wheels, and conventional buffing equipment and compounds. This whole manual process used about five minutes of labor per tack. However, that result, 12 tacks per person/hour, was unacceptable. It would have made my bid for the job non-competitively high. Hand finishing worked fine for a dozen tacks, or even a hundred, but not for a few thousand.
Ultimately, I settled on a combination of tumbling and vibratory cleaning, followed by light hand buffing. The first two highly mechanized processes made my bid competitive. When you are cleaning batches of metal items, try to adopt a batch mentality and batch approaches.
Keep in mind that in most cases cleaning a few similar items does not demand the very high efficiency that cleaning a large number of similar items does.
The intricacy, fragility, and consistency of the object you are cleaning also make a big difference in your choice of process. Attacking an engine block with solvent and scrapers works, but so does the automated cleaning system of oven baking, shot peening, and jet washing used by many automotive machine shops. The latter, however, is simply more thorough, faster, and just plain better than poking at an engine block with solvents and scrapers. It also has the important added advantage of relieving stress in the block’s metal. Of course, there are many good approaches between those two extremes.
The kind of metal and its condition are central to your choice of the most effective cleaning approach. Ferrous metals, including steel and iron, usually take quite a beating if they are not compromised by severe corrosion or structural stress damage. They can be subjected to physically and chemically rough processes and survive intact. However, thin sections of steel, and particularly metals, including copper, aluminum, brass, and bronze, tend to be more delicate and do not take much cleaning abuse. Badly corroded steel (also known as French lace) must be cleaned very delicately to avoid damaging it beyond the possibility of repair. The same is true of badly corroded aluminum and brass, except that they often tend to be even more brittle and fragile than damaged steel.
An item such as a thin steel, brass, or bronze nameplate that is in good condition is inherently delicate and must be treated as such. Hand brushing with a soft brass or stainless-bristle wire brush, chemical cleaning, ultrasonic cleansing, or soda blasting may be the ticket to get such a piece clean enough to repair and/or refinish properly. However, blasting with a peening media, including glass bead, is likely to stretch and warp it, while attacking it with an aggressive abrasive blast media such as aluminum oxide or silicon carbide would probably be unnecessarily violent and might warp or cut through the item. Blasting with an agricultural media, such as pulverized walnut shells, peach pits, or corncobs, is slow, but mild and very useful for dealing with small, fragile parts. Tumbling and vibratory approaches are also possible in this case.
Coatings over clean metal are the first line of defense against corrosion. However, when painted surfaces are as compromised as the one shown here, the coating does no good and can accelerate the rusting process by creating protected cells under which rust propagates.
It is critical to keep in mind that each individual part, or batch of parts, has to be considered separately with regard to many factors (that include size, type, material, and condition) when you choose a cleaning process, or processes. Many situations benefit from the application of multiple cleaning processes. Often these processes are staged to enhance a part’s safety and the cleaning result.
What you are removing from metal can be as important as the size, type, configuration, and condition of the metal itself. Contaminants that sit on the surface of metal are easier to deal with than those that penetrate into its intricacies and pores. With the possible exception of extreme stress, corrosion is the foremost enemy of metals, and is far more common than stress damage. It is a product of metals’ natural degradation, involving the tendency to combine with oxygen to form oxides. These oxides are capable of penetrating deep into and below metal surfaces and into their granular structures. In the case of steel it’s called degradation “rust.”
Welding over contamination causes defects. The top of this weld was made in cleaned metal, while its bottom was welded through dirty, rusted metal. The dirty weld has visible floating contamination in it. Further-more, amperage variations caused by welding over contamination have bulged the surrounding metal.
Because buffing is cleaning, buffed parts such as this nameplate may seem to be safe from contamination, except buffing wax. However, if you get fingerprints on them before you can coat and protect them, those fingerprints may develop under the coating (like the ones on an FBI wanted poster).
This weld was allowed to age without protective coating. It shows rusting in the heat-affected zone (HAZ). That is the area from the weld out that absorbs enough heat to affect the surrounding steel. Note that the visible rusting in the HAZ is worse than that on the weld itself.
In a more general sense, corrosion is an example of the process of entropy, as described by the second law of thermodynamics. It states simply that everything in the universe tends toward a lower state of energy, sort of like a clock running down. You probably never thought of the rust that keeps trying to devour your classic Ford, Chevy, or Mopar in that sense.
Corrosion is not the exclusive disease of steel and iron. Other metals also oxidize, but without showing telltale signs of corrosion. It comes in the form of the red flakes, pits, and powders on the surfaces, the things that you associate with rust. Aluminum oxide is the product of the corrosion of aluminum and its alloys, and is white/gray in color with a powdery texture. It is every bit as deadly to aluminum alloys as rust is to ferrous metals. Copper produces a green oxidation product. Other metals show other characteristic signs of oxidation. A few metals and engineered platings do not corrode under normal conditions, but these are uncommon in the general run of automotive metal surfaces. Some of them are used to plate ferrous metals to protect them. Cadmium and copper/nickel/chromium plating is commonly used for this purpose and to highlight some trim parts.
Non-magnetic stainless steel and hexavalent and trivalent plating of metallic compositions show notable resistance to corrosion. However, these are way off the beaten track of the metal surfaces that you are likely to find on automotive parts and panels.
Corrosion may be confined to just the surface of a part or panel. Superficial rust is relatively easy to remove and to prevent from recurring. Deep, pitting rust is another matter. It is difficult to eradicate it and prevent its recurrence. That’s because metals have granular structures. Rust and other corrosion tend to form along the lines of grain boundaries. Once rust travels below the surface and deep into metal it becomes much more difficult to deal with, but not impossible.
Although rust is the contaminant most likely to burrow deep into metal’s pores and granular structure, it is not the only contaminant that must be eradicated to achieve clean metal surfaces. Paint coatings must often, but not always, be completely removed to refinish auto body panels and parts. Grease, oil, and, silicone have to be removed from metal surfaces for painting, welding, soldering, and plating to adhere properly to them. If you try to apply paint over these impurities it fisheyes, at best. If a finish manages to cover them at all, it fails to gain proper adhesion and fails.
Silicone is particularly irksome in this regard because you cannot see it and it can be difficult to remove. Even minute amounts of silicone combat paints’ surface tension and ability to cover. Solder does not wet properly over oil, grease, and silicone, so proper tinning of surfaces becomes impossible.
Welding over contamination is in a class by itself. Okay, some welding electrodes are labeled for use on dirty and/or corroded metal. They may be capable of wetting and beading on such surfaces, but that only wins a battle, not the war. The more that you learn about hydrogen embitterment and other crack causing phenomena in weldments, the more implicated are impurities that release hydrogen and/or sulfur. Welding dirty metal is an invitation for later cracking caused by hydrogen embitterment or sulfur contamination, among several other bad possibilities.
The cleaner the metal, the better your results will be. The reasons can be complex, but the improved results that clean metal surfaces provide are often visible and dramatic.
No one-size-fits-all prescription exists for metal cleaning processes, or how far to take them. Different jobs call for different mandates. Painting and plating are often the processes that are most intolerant of contamination. Welding is a bit more tolerant at first, but in the long run contamination can come back to bite you in the form of failed welds and cracks in the heat-affected zone (HAZ) adjacent to welds. Buffing and polishing seem to be less discriminating because they are also cleaning processes and often remove detritus that other cleaning processes have left behind.
Then, too, the standards for cleaning the metal used in your projects vary with the extent of the contamination. Here, as you might expect, rust is your toughest opponent. Although dirt, grease, paint, plating, and most other contaminants sit on the surface of metal or interlock mechanically with its surface nooks and crannies, rust is the result of a chemical reaction, and can burrow deep into metal.
In the case of some metals, such as diecast zinc, rust can form from impurities (lead, in the case of diecast zinc) in the metal. These impurities can be below the metal’s surface, so the rust can originate from the inside, out. All of that makes rust and other forms of corrosion much more difficult to remove than most other contaminants. This also makes it harder to prevent their recurrence once they have started to fester.
Now, I need to present a small but necessary bit of rust chemistry. This will be almost painless, and definitely not a chemistry lesson. I promise.
For rust to occur, three conditions are necessary: (1) A substrate must be present. That’s a surface to rust, and is a given. (2) A source of oxygen must exist as well as a medium to allow its transit to a site for rust to occur. Water, atmospheric moisture, and electrolytes, such as water contaminated with road salt, fit that bill and are ubiquitous. In the world of corrosion, you can think of electrolytes as “water on steroids.” (3) A circuit must exist to move the electrons that make the conversion of metals into their corroded oxide forms possible. In the case of rust in iron and steel, this is the conversion from Fe to Fe2O3; that is, from iron to iron oxide, or rust. (The chemistry is actually a bit more complicated than this and specific sequences are involved, but it roughly describes the conversion from iron to iron oxide.)
Because metals are, by their nature, electrically conductive, a circuit through them is another given.
So, the metal is a given and the circuit through it is another given, leaving only one foundation for rusting that you can manipulate: the presence of moisture or an electrolyte. The only practical way to stop rust is to deprive it of water, moisture, and, most important, electrolytes such as salty water. Okay, you have the basis for a plan. But, of course, there are several complications in implementing it.
The first is that the substrate has to be incredibly clean before you can even consider depriving it of moisture. Here’s why: If you leave any corrosion on a surface that you later coat, it contains moisture absorbed by the corrosion from the atmosphere. That moisture causes the substrate to rust further under your paint or other coating.
That’s critical because rust expands to roughly 17 times the volume of the iron or steel from which it forms. Other metallic oxides have similarly disastrous displacement rates.
Why do I call them disastrous? Because metal oxides exert enormous pressures as they expand to occupy their rightful space in nature. They literally push coatings, including paint, off substrates with the greatest of ease.
This panel surface was sandblasted and it then sat, unprotected, for a month. Rust specks and streaks have already appeared on it. If you try to prime and paint over them, you do so at great risk. The problem is what lies below the visible beachheads of rust.
In this drawing you see a little rust spot on top of a steel surface, penetrating deep into the metal’s granular structure. The penetration follows an electrical circuit along the metal’s grain boundaries. This makes it very difficult to eradicate the rust completely. Small rust specks can foretell big problems.
To make matters even worse, coatings, such as paint, have limits of elasticity and adhesion. If you push them far enough to stretch them beyond those limits, they fracture and detach from substrates. When they fracture, the fracture lines, literally cracks in the paint, tend to act like little capillary pumps that pull water and electrolytes in under the coatings where they have detached from surfaces. This adds moisture and/or electrolytes to the rust stew that is already brewing under the coating from the original bit of corrosion that was coated over. Then, the rust festers and expands some more, using the electrical circuit(s) through the metal’s granular structure that was established by the original rust that was left there under the coating.
The cracks in the finish get bigger and more cracks develop in the coating. More oxygen-bearing moisture enters through them. At this point, you have a corrosion cell, a veritable rust generator, under your coating. There is no way that this little mess can do anything but get worse, and worse, and worse.
Ospho is one of many available metal preps and conditioners. I’ve always had good results using it to protect unpainted steel surfaces until I can apply a finish to them. Ospho improves paint adhesion, too, by etching metal surfaces. Always rinse Ospho off with water before it dries.
This is how and why pinhead size rust spots under paint grow to dime, quarter, and then fifty cent size sores on their way to destroying whatever they are attacking. If this sounds like the scenario for a Saturday night horror movie, it might make a great one. My choice would be Bela Lugosi or Boris Karloff to play Count Corrosion.
Do not abandon hope. In this epic battle between good and evil, well, between rust and us, remember that we are free men and women with free will, intelligence, and resolve. We can analyze the situation and fight back with proven counter-measures. We can win. We will win. Here’s how.
With those two “givens,” metal and a circuit through it, and one “ubiquitous,” water, moisture, and/or electrolytes, you can have only one possible plan of action, and some strategies derived from it. Your plan must be to deprive the substrate of the things that give it the oxygen that causes it to produce rust, and other oxides in other metals. Foremost among those things is water and water containing electrolytes. Your plan is to deprive substrates of these evil rust nutrients and cut the rusting process off at the pass.
Implementing this plan means removing all rust from the substrate metal so that nothing is left there to hold moisture and to promote new rust under the coatings that you place over substrates. That is the first strategy. It can never be accomplished perfectly, but the cleaner you get your base metal, the less likely it is to rust after it is coated.
The second strategy is to either convert the clean substrate to something other than rust that is so chemically stable that oxygen is unlikely to displace it and then combine with the substrate metal to create new rust or other undesirable oxides. “How do I do that?” you ask. You can do that by converting it to more desirable oxides.
Every major paint company offers one or more reactive (etching or self-etching) primers. DuPont’s Vari-Prime is a two-part epoxy self-etching primer that provides a terrific foundation for finishes when applied over clean metal. Never use any reactive primer over a rust converter, conditioner, or prep such as Ospho.
The third and final strategy is to coat either the clean or converted substrate with a coating that moisture and electrolytes cannot easily penetrate.
Most of this book concerns the first strategy, making metal so clean that there is not enough remaining corrosion to start a corrosion cell. That is always the first-line defense in preventing corrosion from attacking your work.
The second strategy, converting the surface of the substrate to something that rust does not attack, has several branches. In the past, so-called “conversion coatings” and “rust converters” employing phosphoric acid, tannic acid, and other agents have been the most usual way to accomplish this strategy. These coatings work very much like the gun bluing that converts iron and steel surfaces to Fe304, a stable blue/black finish that protects against rusting. There are many conversion coatings on the market. Automotive paint manufacturers offer some of them as parts of anti-corrosion finishing systems. These are often called “metal conditioners” and “metal preps.”
Aerosol self-etching primers are widely available and much less expensive than the automotive paint company products in this range. Although they are less effective and durable than those products, they still manage to do a very credible job, and with much less application fuss and expense.
Another class of products that converts steel surfaces to more stable oxide forms is reactive primers. These are usually epoxy based and are available in one- and two-part formats. They are designed to coat and react with substrates to form mechanical and chemical bonds with them. This produces a conversion of the surface under the coatings to an oxide of steel/iron that is not the common rust degradation oxide for the metal. Reactive primers also tend to be very molecularly dense and, therefore, resistant to the transit of water molecules through them. (Conventional sanding primers do not have that kind of density and offer virtually no protection for base metal until they are top coated with denser finish coats.) Reactive primers provide another way to deprive substrate metals of the oxygen that they require for corrosion, while changing the metal in the surface to a format that strongly resists corrosion.
Slow-dry enamel primers and paints, such as the legendary Corroless, encapsulate and isolate rust. The deservedly highly reputed Rust-Oleum products also do a good job penetrating and encapsulating minor rust, and preventing its propagation. Fortified slow-dry enamels such as these have many good uses but are not suitable as automotive topcoats.
It should be noted that conversion coatings, metal preps, and metal conditioners should never be used under reactive primers. Each member of this class of anti-corrosion treatments and coatings is a sole measure that cannot be combined with other chemically reactive metal conditioning and coating measures.
The third strategy is to cover the cleaned and/or converted metal surfaces with coatings that are so impervious to the transit of water molecules that even without converting their surfaces to a more stable form than the base metal, the coating protects them from corrosive molecules. Moisture-cure urethanes are notable for success in this mission. They are also very tough and resilient to abrasion. However, they may lack strong resistance to ultraviolet light, and they do not possess the physical qualities to make them good topcoat finishes. Their resilient toughness results from their softness, making them difficult or impossible to sand as primer coats.
A variant of the impervious-to-moisture coating strategy involves using products that isolate and stop rust without converting it, and then encapsulate the rust to prevent moisture from getting to it and causing further rusting. The best known in this subset of barrier coating products are oil-based slow drying enamels. Some of these have origin in exotic settings, including coating the insides of pipelines. Others are fairly common and can be found on the shelves of your local hardware, building, or farm store. These products are fortified with various additives to help them penetrate light rust and neutralize it. Some of them work very well, but none produces an automotive-quality finish. However, if you are painting something like a water pump, bellhousing, or flywheel cover, these products work quite well and can tolerate small amounts of rust under them for surprisingly long periods. They are the coatings of choice if you cannot resist the temptation to paint over a speck, or speckles, of rust.
Clearly, the first and most important step in preventing corrosion is to start with the cleanest possible surface, before any coating is applied over it. That is the basic defense and gold standard of fighting rust. Conversion coatings and reactive primers offer further insurance against corrosion. They should be seen as very useful secondary strategies in the war against rust. Fortified slow drying oil-based enamel is another tool for fighting rust in some situations.
We know that clean metal is critical to success with metal projects. But the question remains, how clean does metal have to be? The simple answer is, as clean as it is practical to get it. That means that you should do everything you can reasonably do to remove contaminants from metal surfaces and to prevent them from being re-deposited on those surfaces before you convert and/or coat them. That sounds pretty flat-footed simple, and it is. Regardless of how good your secondary lines of defense against corrosion are, your primary defense is always clean metal. The temptation to leave a speck of dirt or rust on a surface, and hope that your conversion coating, reactive primer, or slow dry enamel takes care of it is, most often, a temporary fix and an illusion. However, truly clean metal is virtually unattainable in some places and situations.
Take, for example, the crimped door skin or decklid seams at the edges of those panels. From a corrosion/clean metal point of view they are disasters waiting to happen.
In many climates, these seam areas are subjected to the winter assault of one of the world’s truly great corrosion electrolytes, salty water. It is propelled toward them at high velocity in aerosolized form by the tires of oncoming and passing vehicles. Moisture condenses inside these crimped edge panels and combines with the dirt and debris that has become resident there. This crud-laden brew runs down the panel’s insides to their bottoms, where it sits on the inside tops of the bottom crimped seams.
Typically, dirt and debris have already collected there. And there they act like sponges to hold liquid contaminants against the inside of the crimped seams. What gravity and vibration don’t do to draw this awful mishmash further into the seam, capillary action accomplishes. And, of course, the assault by salty water is simultaneously taking place on the outsides of these crimped seams.
Now, there’s a whole galaxy of new things to worry about as you drive through the snow and slush.
Without taking such seams apart, no practical way exists to determine if the metal in them has been adequately cleaned. Worse, you have no way to determine that corrosion hasn’t started to attack them, unless, of course, the area is already so badly and visibly damaged that it is necessary to apply a new panel skin, or to section in new metal to replace the old seam metal. Any method of cleaning that goes deep enough into the seam to eradicate most of the potential rust there, such as a chemical immersion approach, tends to leave residues that are extremely difficult to remove or neutralize. That kind of contamination can lift any coating that you try to apply over such crimped seam areas.
In this case, the best solution is to get the area as externally clean as you can, and then to seal it as completely as possible against the further entry by water and electrolytes. Moisture-cure urethanes that come in liquid and paste formats (paints, “seam sealers,” and caulks) are good for this because they are relatively impervious to transit by water and other liquids, and they tend to leach moisture from substrates as they initiate their cures. Anti-corrosion sealing products, such as foams and gels (that are not based on moisture-cure sequences) claim to displace water and moisture, and to seal against their re-entry. Hot-applied undercoating products based on paraffin waxes and fortified with corrosion inhibitors can also be very effective in these situations.
The only sure way to stop the rust in this decklid seam is to remove the skin and replace it, or to repair its damaged areas with new metal. Cleaning the area thoroughly and applying moisture-cure urethane paint or seam sealer to the exposed outside joint may work temporarily.
Happily, areas such as panel edge seams are exceptional in the difficulties they present in cleaning and sealing. Most surfaces on cars are easier to inspect and clean. Take, for example, the outer surface of an auto body panel, including a fender or a hood. The metal in them is almost completely accessible, either mounted on a car or removed from it.
How clean does a fender or hood have to be before you can weld or paint it? Here’s a hint before you try to answer that one. (“Perfectly” isn’t the right answer, because that is an unattainable state in the real world of cleaning.) In fact, that fender or hood surface doesn’t have to be “laboratory” or “metallurgically” clean to weld it or to have it hold paint reliably. But it will likely need to be as clean as you can get it for those purposes.
Seam-sealing products such as these are designed to seal crimped seams. The two on the left from POR 15 are intended for application over rusty metal. They leach moisture out of rusty seams and protect them with a dense, waterproof coating.
Solvent parts baths such as this one are mainstays for cleaning mechanical parts. They perform well in that role if sediment is removed from them regularly and their solvents are replaced when they become contaminated. They should never be the final step in cleaning for painting.
How clean something looks and how clean it is are usually critically different. Paint and rust are at the easy end of the scale of contaminants that you have to remove from metal. You can pretty much see where they are and when they have been mostly removed. Or can you? If you carry your inspection to a microscopic level you often see remnants of things that you thought you had thoroughly expunged.
The 3D microscope in the center lets you look at metal surfaces far beyond what you need to inspect for decontamination. However, the small, handheld, illuminated pocket microscope (left) and the illuminated magnifying glass (right) are great tools for inspecting metal surfaces for cleanliness.
Then there are contaminants like oil, grease, and silicon. They tend to be invisible to the unaided eye. Yet any of them in small concentration can cause paint to fisheye and/or to lack adhesion. Even the fingerprint of anyone guilty of touching unprotected metal deposits enough body oils and moisture to become a potential cause of painting defects.
Some contamination does not attack surfaces from the outside; it lurks beneath them. For example, lead impurity in some diecast zinc items, such as trim and door handles, can corrode these parts from the inside out. In this case, corrosion cells start beneath the surface and work out. Removing all traces of corrosive contaminants from these parts’ surfaces and subsurfaces is challenging in the extreme.
The truth is that while you can never get anything absolutely clean, it doesn’t hurt to try, even if you never achieve it. At best you will dilute contamination from the surfaces of metal to levels that are not a problem for your next steps: welding, painting, plating, etc. This doesn’t sound very elegant or hopeful, but it is very serviceable. Get metal surfaces as clean as you can, but realize that there is always something left lurking on them. If you dilute that something sufficiently, you have used a tactic that will probably win your war.
In the case of rust, you may leave some of it behind, but if you convert the metal surface to a stable form that is not rust, you have again used a tactic that gives you an excellent chance of winning your battle with corrosion.
Metal wiping solutions such as DuPont Prep-Sol do an excellent job of removing oil, grease, and particularly, silicone from metal surfaces. Less expensive fluids including trichlorethane-based brake cleaners and enamel reducers also remove oil and grease but do not remove silicone residues as completely as do wiping solutions from paint companies.
Use a metal wash to remove oils and silicone before you apply a metal converter solution or etching primer; it will likely save your project from defeat by some insidious and largely invisible enemy of paint adhesion.
Barrier coatings such as moisturecure urethane paint paste or caulk leach moisture out of a substrate and are great weapons in your battle.
Employing a slow-dry enamel to encapsulate and isolate rust is also a serviceable strategy against corrosion.
None of these weapons or tactics results in outright, automatic, or irreversible wins, but if they allow you to take enough of the rust enemy prisoner, you will likely win the war with corrosion for a very long time.
