Prep the metal? I don't know about the 99's, but I know for a fact that the rear bumper on my 2001 is plastic. You shouldn't have to worry any about prepping before applying touchup assuming you didn't crack the primer off as well... Just remember to put touchup clearcoat over your colored paint when you're done to minimize the visible difference. For small little dents and dings, you should be fine. As far as that bigger one you were talking about, though, I don't know...