I usually do door jambs and undersides of hoods with
3M Scotchbrite pads.
I use a red one if I'm going to prime over it,
If I'm not priming it, I use a gray one (finer scratches)
I usually wash and scrub real good useing Dawn dish soap
on the pad. Sometimes I'll use Ajax cleanser but always a final
wash with Dawn, dry real good and it's ready for paint.
For a more durable finish, I'll use a seal coat before paint
of reduced epoxy (50/50) before base or urethane.