As I'm sure many of you remember from being 15, getting your hands on a welder and sheet metal can be a pain. So, my question is as follows: Can I JB Weld small pinholes and expect them to last long enough to not rust further until I get extra sheet metal? I am very well versed in fiberglassing, but I have heard if I try to make fiberglass parts of panels, they will expand to fast, and drop out of place from the original part. Is this true, and is there anyway I can stop this behavior?
First off, you are asking about a temporary repair until you can replace the rusted area so let's address the your question.
Rust, unless protected from air and moisture will continue to spread. If you can gain complete access to both sides you can treat the rust and stop it's spread. Chances of this are slim so no matter what you do it will eventually fail. If you can access the back of the rusted area, even if not completely, you have nothing to lose by treating it with POR 15 or Eastwood Rust Converter.
The quickest, simplest way to make the outside look right would be to grind the metal, tap it in to form a depression and patch it with a fiberglass impregnated filler such as
THIS. Don't let that price scare you, it's also available in smaller cans.
This will last for a while - how long depends on how well you treat the rust on the inside.
I remember what it's like to be 15 with a limited budget and skills, so that's my best suggestion considering the circumstances and your original question.
