One option not mentioned is products like Metal to Metal, I got this from Eastwood CO. Bondo bubbles from moisture usually entering from the back of a repair like a weld. I try to only do butt welds, utilize light from behind to help detect and fill all weld holes. I then use Metal to Metal on the outside of any weld seem or repair, it is applied like bondo but is an aluminum paste with fiberglass catalyst, it is impervious to moisture. Metal to Metal can be sanded easily and have primer or bondo applied over it. I have used this product with great success and none of the mess and danger of lead. Good luck
