You talking a bench grinder setup with a wire wheel?

Bench grinders and buffing motor are two different things. I used an old 1/3 hp sump pump motor and eastwood buffing kit on my car. I had great results.

Some wire wheels are tougher then others. Some can really gouge things up.

Best black paint I have found for engine compartment stuff is the SEM Products 39143 Trim Black Aerosol.

I usually strip parts down to bar metal by either media blasting, chemical stripper or wire wheel. I soak rusty parts in tub of Evaporust(harbor freight) or phosphoric acid(Home depot). Wipe the part down with some surface prep and finally sweat the metal out with a propane torch before I spray it. Works pretty good for me.