Well, you're gonna want to make sure they are very clean, otherwise the paint will not stick in areas (aka fish eye). Chances are someone along the way has Armoralled the crap out of 'em. You can wipe 'em down with vinegar then rewash with good old Dawn dish washing detergent in water. After that you can wipe it down with either a commercial prep cleaner (Cleans-eze [napa]) or a little enamel paint thinner on a rag. After that, tack cloth them to get up any dust and paint away.
Hope you find this helpful.