Simon. The important thing is to get the panel welded in solid and the metal worked so that it is rigid. If it looks like you will have quite a bit of filling to do, use the fiberglass reinforced filler for your initial coats. The final coat, in my opinion, should be regular filler that covers the entire area. I use extra resin in this final coat of filler to help with eliminating pin holes and to make it easier to spread evenly. Try to finish this final coat with 80 grit and 180 grit paper. If you work it too long with 40 grit, you will leave heavy scratches that will have to be fillled with primer that may shrink later.