luckily all the body work I’ve done is always on a removable body panel. But if I wanted to fiberglass a large rusted crack, how would I go about that? Since I can’t get to the backside of the panel. It’s for a 64 el camino. Normally I would get a patch kit and just weld the new piece on. But I can’t find a patch for the rear quarter panel.

Thanks for your time. I hope I’m not just overthinking an obvious thing here.

Well, that's a tough one....if you can't get to the back of the area fiberglass is not an answer. Hopefully a few more guys will swing over here for some input..... Pictures of just what you are working with here would be helpful....

I had a similar problem with the 1965 Buick Special Convertible (see projects section) and ended up going with a used quarter panel.

Did you try these places:
https://www.c2cfabrication.com/collecti ... parts/1964
https://www.opgi.com/el-camino/1964/she ... ody-panels
https://www.elcaminostore.com/exterior- ... parts.html

Thanks for the links! I’ll give those a look.

Yeah, that's no place for a fiberglass fix.....hope you can find a panel for it!