If you can't weld the holes a good fix is to use marine epoxy to fill or cover from the back side. It works with fiberglass matt (thin) or cloth and for the most part marine grade epoxy is water proof. Sand or grind and bondo sticks to it just fine. I get mine (POXY-PRO I think) at Menards. It seems to hold up better than fiberglass fillers as a moisture barrier and it's handy around the shop.