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i've never painted an engine before this is my first out-of-the truck major rebuild and i thought i might as well make it look nice too. it's a 1969 429
Make sure you degrease everything good or the paint will not stick. Be sure to mask and plug any holes before painting. Do not paint any mating surfaces where gaskets go. I like to paint the parts separte before I put the engine back together (heads, block, oil pan, valve covers, intake manifold, brackets, pulleys, water pump, alternator, AC compressor, etc.). I also like to use a high temp. primer in the block valley to promote better oil flow. Make sure you use an engine temp. rated paint. Unless you are doing alot of engine painting, I would use spray cans (I like DuplaColor) and not an air system. Get one of the handles that go on the top of the can and has a trigger. It makes control of the paint alot better. As with any painting Prep, Prep, Prep, is the most important part of the process. (clean parts and surfaces, degrease with a good paint degreaser, mask, tape, & plug).
I've painted alot of engines, but never got very good at it. Next one I do will be done with epoxy primer first, then with a bunch of coats after. My guess of where I have failed is with the prep, like plowhand said. Oil and grease being the major problems. I also suspect the tiny peaks from castings, they don't cover well and therefore start giving up quicker.
Clean is important. I have started to use Rustoleum barbeque paint. I know not much of a color selection. However it sticks, and you can them apply your color over it. Also for exhaust and headers the BQ paint works better than header paint. Seems to be more tolerant.