Make sure the metal is always clean. If you do patches straight away then no pimer is needed but misture gathers on the metal fairly quick. Weld thru primer is good and is what stops the metal from bubbling out again. I found out (from my body guy) that most of the amateur body guys fail by not putting epoxy primer down on the clean metal then the fiberglass filler goes on. Never put bondo on clean metal as that is how it bubbles again. And another caution - epoxy filler will absorb mooisture as well so get the bdy work done then use a good sealing primer if you want your car to look good for years to come. Mine goes through rain and is 8 years old paint and still looks new.
