I did some welding on my rear valance to replace a rusty area. I used zinc-chromate weld-through primer on the backside of the patch panel and on all the welds after. I would like to use a bit of skim coat finish bondo to smooth out one area. Can I skim coat right over the weld-through primer or does the area have to be bare metal? What's the rule of thumb for bondos and skim coats after body work?