How to paint an engine

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.

Painting an engine can be done either before assembly or after, it really does not matter. Our 302 block had been machined and is ready to assemble, so we just painted it up before assembly. The engine was wiped clean with paint thinner and then wax and grease remover.

Painting an engine can be done either before assembly or after, it really does not matter. Our 302 block had been machined and is ready to assemble, so we just painted it up before assembly. The engine was wiped clean with paint thinner and then wax and grease remover.

 

Since the motor is not assembled yet and still has the old paint on it, some serious cleaning was needed. The plan is to soda blast the block. Baking soda is great for engine cleaning and it is ok to get it inside the engine, but keeping it out is pretty easy, so we taped up the engine. First, a couple of layers of masking tape for the base.

Since the engine is not assembled yet and still has the old paint on it, some serious cleaning was needed. The plan is to soda blast the block. Baking soda is great for engine cleaning and it is ok to get it inside the engine, but keeping it out is pretty easy, so we taped up the engine. First, a couple of layers of masking tape for the base.

 

Then the main cavity was covered up with newspaper, we used two layers here. The piston bores and lifter valley were taped up in the same manner.

Then the main cavity was covered up with newspaper, we used two layers here. The piston bores and lifter valley were taped up in the same manner.

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.

We took the engine outside and set up the blaster. For this type of cleaning, all you want is soda. The blaster uses a mechanical dead-man valve to shut off the flow of media. While great for small projects and value, the heavy spring on the valve leaves your hand worn out. For small parts, like this engine, it worked great. Note that all of the holes in the engine were taped up.

We took the engine outside and set up the blaster. For this type of cleaning, all you want is soda. The blaster uses a mechanical dead-man valve to shut off the flow of media. While great for small projects and value, the heavy spring on the valve leaves your hand worn out. For small parts, like this engine, it worked great. Note that all of the holes in the engine were taped up.

 

Because soda is very soft, tough spots like inside corners, pitted areas and edges are more difficult to blast, requiring more effort on your part. Baking soda has a hardness of 2.5 on the Mohs scale, while the hardness of engine bearings is around 6, so even residual soda left in the motor will simply be absorbed into the oil (or water) until you change it. No harm, no foul.

Because soda is very soft, tough spots like inside corners, pitted areas and edges are more difficult to blast, requiring more effort on your part. Baking soda has a hardness of 2.5 on the Mohs scale, while the hardness of engine bearings is around 6, so even residual soda left in the engine will simply be absorbed into the oil (or water) until you change it. No harm, no foul.

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.

11. Soda blasting leaves a residue on the metal, which must be neutralized. Skipping this step would be akin to soaking the engine is oil before painting, the paint will just fall off. We used a vinegar/water wash to clean the prepped engine. You know it is working by the bubbles. Then we installed the heads without gaskets. These are only hand tight, no torque.

Soda blasting leaves a residue on the metal, which must be neutralized. Skipping this step would be akin to soaking the engine is oil before painting, the paint will just fall off. We used a vinegar/water wash to clean the prepped engine. You know it is working by the bubbles. Then we installed the heads without gaskets. These are only hand tight, no torque.

 

Next, the engine was taped up so that the only areas we wanted painted were exposed. This means gasket surfaces and hose areas get taped.

Next, the engine was taped up so that the only areas we wanted painted were exposed. This means gasket surfaces and hose areas get taped.

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.

Using a measuring cup, we mixed only enough Napa base color to fill one cup for our gun. The base is a 1:1 mix, with these cups, it is hard to go wrong. A smaller cup would have been nice.

Using a measuring cup, we mixed only enough NAPA base color to fill one cup for our gun. The base is a 1:1 mix, with these cups, it is hard to go wrong. A smaller cup would have been nice.

 

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.

The gun used for this job is a Devilbiss detail gun. The smaller tip provides a smaller fan, so you don’t get as much overspray. This is the perfect gun for this job. A big gun will work too. Spraying paint on an engine requires a light touch. You want just enough paint to cover the surface without being splotchy.

The gun used for this job is a detail gun. The smaller tip provides a smaller fan, so you don’t get as much overspray. This is the perfect gun for this job. A big gun will work too. Spraying paint on an engine requires a light touch. You want just enough paint to cover the surface without being splotchy.

 

The engine was rotated to give access for the underside. There are a lot of nooks and crannies, these need to be reached. You could easily leave the engine like this, though the paint would require mixing with hardener before painting. It looks good, but not quite ready.

The engine was rotated to give access for the underside. There are a lot of nooks and crannies, these need to be reached. You could easily leave the engine like this, though the paint would require mixing with hardener before painting. It looks good, but not quite ready.

 

24. The silver metalflake was sprayed through the gun with a large 2.0 tip. We had to be careful not to put too much in one area. Once we had the metal flake on the motor, we sprayed the last coat of clear. Again, this was light but solid coat, covering all surfaces.

The silver metalflake was sprayed through the gun with a large 2.0 tip. We had to be careful not to put too much in one area. Once we had the metal flake on the engine, we sprayed the last coat of clear. Again, this was light but solid coat, covering all surfaces.

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.

Once the paint had cured, it was demasked and then the engine was assembled. Here is the finished product.

Once the paint had cured, it was demasked and then the engine was assembled. Here is the finished product.

Check out all the paint & body products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to paint an engine, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

about author

Jefferson Bryant

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.

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1 Comment

  • BRETCO

    September 5, 2016 at 8:39 PM

    Reply

    Cool man, clear straight to the point, well done. The motor looks bitching.

    I just happened to be sitting here thinking how to paint the motor I just pulled out of the Mustang, how and what color? Did you still use the high temperature paint? Sure would open up the color chart. Thanks for your time. Good info.

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