Builder,
That "black" primer you are referring to is actually an EDP coating. This coating is used to prevent rust. You don't have to remove this coating as it keeps the metal from rusting. My experience with this coating is to scuff it with 320 grit, primer it, let it dry, 400 grit, primer sealer, let it dry, 3m scuff pad, clean with tack cloth, then a grease and wax remover and tack again. Then paint it as you will.
Tim
