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