Once in a while I get some farm equipment to paint and recently we were hired by a neighbor to paint his Ford tractor. The first step was to get the tractor in the shop, take off the wheels, fenders and hood then clean it well.
The engine was quite dirty so we removed the battery and cleaned it will using Super Clean. This stuff really cuts the grease but be careful not to breath it. Whew!
After cleaning we scuffed everything with a combination of a red scuff pad and 320 sandpaper. In the places we couldn't reach (like some areas of the engine compartment) we just cleaned and applied Evercoat's adhesion promoter prior to painting.
After all the cleaning and scuffing we masked for painting. The wheels and fender got painted first so we could get them out prior to prepping the rest of the tractor.