My ’79 van had urethane foam insulation sprayed all over the interior of the roof and walls. 35 years later I’m finding several holes rusted thru from the inside out. The damaged areas are rusted in layers of black decayed metal that are wider from the interior side goes deeper until a small hole erupts outside on the rooftop. My guess is that it might be from moisture trapped inside somehow.
I haven’t ground out the rotted metal to open up the hole to it’s maximum size but I’m guessing some of them might be as large as 2″ long by 1 1/2″ wide.
I suppose the best thing to do would be weld in patch panels. The holes will be too large to just weld shut. However, I’m not an experienced welder… Is fiberglass-strand bondo strong enough to patch holes this size or would it be inappropriate?