How To Paint an Engine
Engine heat wreaks havoc on paint, even the paint on the hood can fade from the heavy dose of radiant heat from the hot engine below. How well do you think that rattle-can paint job is going to last? We have sprayed engines with spray cans, even the high-heat versions, only to see cracks, flakes and fading in just a few weeks of use. Take a look at any high-buck build and the engines are painted up to match, but they don’t fade, crack or flake, how do they do it? How to paint an engine so that it lasts is all in the method. Rather than take a chance with a rattle can, we did some research to find out what the best method is for a long-lasting engine paint job. After spending some time talking with various restoration and custom shops, we had our solution.
There are two problems with painting an engine- adhesion and heat. The first is relatively simple to solve with hot-tanking, scrubbing or in our case, soda blasting. We spent a couple of hours tediously taping up the engine for soda blasting. In the end, the tape didn’t keep the soda out of the block and according to several soda-blast resources, is not even a problem. Baking soda readily absorbs into oil and water, and the hardness of the material is below that of even the softest bearing surfaces, so taping before blasting turned out to be a big unnecessary hassle. If you are painting an assembled engine, the best bet for the prep work is hot soapy water and lots of elbow-grease.
The heat issue is the biggest problem. The enemy of paint is heat. We have all seen how paint bubbles up when heat (like from a heat gun) is applied. These are the same mechanics at work on a painted engine. In the case of rattle-can paints, no amount of heat resistance additives can combat cylinder head heat. The problem is that rattle can paint is too thick. The paint used in these cans is non-catalyzed, which means it has to rely on special solvents to cure. If you have ever sprayed a cold rattle-can, you know that it takes two or three times as long for it to cure. This is because the solvents must vaporize before the paint cures. Thick paint reduces heat transfer, which allows the heat to build on the surface of the engine, instead of wicking to the air; just like spreading butter on a burned finger, it just traps the heat.
Traditional automotive paint, the kind that is sprayed with a paint gun, either single-stage or base\clear, is cured with catalyst. While these paints still use solvents, the solvents dissipate much faster than rattle-can paint, and have an active ingredient that actually cures the paint. In base\clear formulas, the base coat does not have a catalyst, it is instead thinned using reducer depending on the temperatures of the atmosphere at the time of spraying. The clear coat, along with single-stage paints, are catalyzed with hardener. The hardener cures the paint so that it is stable. These paints are thinner and have more even coverage than rattle-can paints. This works to our advantage, as the thinner the paint, the better the heat transfer.
But what about primer? All automotive paints require primer to get adhesion right? Absolutely, if you are talking about sheetmetal. It is true that you would not want to spray a car without primer first. For one the paint would be splotchy from variations in the body work, but also the paint has a hard time sticking to smooth sheet metal. An engine uses more porous materials, like cast aluminum and cast iron. The paint won’t have a problem sticking to these materials, as long as it is clean. The main problem with primer, even basic etching primer, is that it is thicker than the paint itself. Remember, the idea is to reduce the thickness of the paint. So when painting an engine, leave the primer out.
For our small block Ford (a 347 cid stroker actually), the idea was to paint it red and add a touch of metal flake. While the metal flake might not be suited for a resto, the process is the same. We used a base\clear NAPA paint, and sprayed the engine in the shop. Keep in mind that spraying this kind of paint generates a lot of overspray, much more than a rattle-can, so if you don’t want it tinted engine color, cover it. Sometimes you can rent a paint booth for an hour at the local body shop. You also need a respirator with charcoal packs. You really do not want to be breathing the vapors of this stuff. We spent about two days prepping and spraying our engine.
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