Large fender hole

I found some bubbling paint at the bottom of my fender right behind the wheel. When I started sanding, I discovered I actually have a large hole. I will probably use fiberglass to cover it, but any suggestions what I should put in the hole to back the fiberglass? Or should I just fill it solid with some sort of Bondo?

Thanks.

 

[MONMIX]

I would use metal to fill the hole and a good quality body filler to finnish

 

[edhand]

I would use metal to fill the hole and a good quality body filler to finnish

I would love to weld a patch in, but I don't have the torches or experience to do it.

 

[ExplorerDMB]

bondo is not a subsitute. Why they call it "body filler" I don't think they should; just call it "Body Finish" or something. Anyway, Metal is the best way to go, fiberglass possibly - but just make sure you cut all the rust out or it'll return.

-Drew