I'm in the rail car repair business and have access to several different coatings for industrial use. The underside was painted with a DTM (Direct to Medal) epoxy which flow very good. The engine block I painted with an epoxy primer and used a urethane top coat. I mixed it until it matched what was on the oil pan. the fire wall I had mixed with a DuPont formula mixed at a PPG automotive shop. It matched perfectly to some paint that was left under the wheel well. I did paint it with the bushings installed.
This has been a nasty job, lots of gook and undercoating to remove. I have been on it (off and on) for over a month now. I have some components to put back on the engine, the starter, generator and radiator and then I'll post some photos of the engine compartment. I'm sure in all of it I painted something the wrong color or painted something that should not have been painted. Right now I am just proud to get it painted, the block, transmission, the under side, the inside fenders, firewall and trunk. I'd rather paint the outside three times over than do this again.
I am very very pleased with the outcome. Painting is always the reward for hours and hours of prep work. On now to the body and getting it back to a Caribbean Coral and then the seat covers and door panels.