Learning how to paint antique engines can be trying. There is nothing as frustrating as expending a lot of time and effort doing a meticulous mechanical engine restoration and then winding up with a so-so paint job. But through trial and error (with a lot of errors), and talking with people who are professionals with paint, I have come up with a system that works for me.
Make sure it's clean
First of all you must get the base metal clean. This usually starts for me with a trip down to an automotive machine shop that has a hot tank. For a thorough cleaning you can't get much better than this, and you save yourself the mess of this chore and you don't have to deal with noxious cleaning chemicals around the garage. Some types of hot tanks will even remove rust. Just make sure that there are not any babbitt bearing or pot metal pieces on what goes into the hot tank as these materials will be dissolved by the cleaning process.
After all the parts have been degreased, I take a wire brush to them to remove all the loose rust and clean the areas that the hot tank cleansing might have missed. Bead or sand blasting will give you even better results.
The next step is surface cleaning to remove oil or foreign substances that would keep the primer coat from sticking to the base metal. I have used several different chemicals for this task, including paint thinner, alcohol, lacquer thinner, and brake parts cleaner. All of these work fine. But my favorite cleaner is a product called Quick Prep from DuPont, the makers of Dulux enamel. You simply wipe or brush Quik Prep on and when it dries you have a clean and oil free surface that is ready for paint-no muss, no fuss.
Applying the primerI like to apply paint with a brush. There are advantages to spraying but I don't have the equipment and I don't like the mess it generates. Begin with at least two coats of primer, allowing at least a day to dry in between. Red oxide primer goes on best when thinned down slightly with paint thinner. I use soft drink paper cups from the local slurp-n'-burp to mix my paint in. Never paint directly out of the can. Too many important volatiles evaporate from an open can, and eventually the consistency of the paint turns to something resembling maple syrup. Clean-up is real easy with these disposable paper cups, and you can save the paint for next day touch-ups by sealing the cup overnight with plastic wrap, making it airtight.
Applying the finish coatAfter priming, apply the finish color. At first I did not like using DuPont Dulux because the paint is too thick and heavy to apply with a brush. But I learned from a DuPont paint dealer that mixing DuPont fast dry reducer (Y-3812S) makes the paint easy to apply and dries practically dust free within an hour.
Here in the dry climate of Colorado I dilute the paint 10 percent with reducer. The reduced paint is very thin and flows easily from the brush. Although the paint is watery in consistency, I do not have problems with runs unless I overload the brush with paint.
Again, allow at least a day to dry between coats. I usually apply three finish coats. Remember that after your final coat it will take the paint a week to become totally tack-free, and up to two to three weeks for the paint to fully harden. It helps to use cotton gloves when you are assembling a recently painted engine to keep from disturbing and marring the soft finish with your bare hands.
As a final note, remember to read the labels on every paint product and to use them only in well-ventilated areas. Painting my engines has become a lot easier since I developed this system. Try it for yourself, and improve upon it until you achieve your best results.