Glyptal engine paint??

I have a lot of experience using Glypal paint inside engine blocks and electric motors. As others have mentioned, prep is everything. On my builds I use a rotary zip tool and a bunch of brass brushes and clean the crap out of everything. Then I use white coffee filters and denatured alcohol and scrub everything until I can't see any more dirt. The white filters make this easy, so obviously don't use the brown ones. The white filters are strong and naturally lint-free.

You must absolutely bake it on after you apply it and let it sit for a few days (or a week) to cure. I have friend who does powder coating and he has a large oven, so that makes it easy.

Of course, if the inside of the cases are perfectly smooth then I don't bother. But if the innards were made with crappy castings and have tons of casting flash and rough surfaces, then I use Glyptal after removing all of that extraneous material.

For example, the aluminum VW/Porsche motors I work with typically have very smooth interiors and generally don't require Glyptal. But the Corvair aluminum engine blocks and various American cast iron engine blocks and cylinder heads almost always have very rough interiors. So those get the attention.

Also note that I could never use Glyptal in a production shop simply because the time it takes to do it right is so lengthy. But for my engines, I like to to an application.

Someone asked why it's even necessary, since oil will roll off oil. It comes about because if the engines are really porous and rough, then the engine oil will carbonize and stick inside those areas, and that's where oil doesn't like to flow. If you need to rebuild your engine with any kind of regularity, or if you have a problem and need to pull your engine apart, or if you have a naturally coke-y engine, it's a million times easier to clean and re-build if it has a coating of Glyptal.

Just my $0.02.