Re: repairing A/C box with fiberglass
As Alan said, the kit with the fiberglass material, the resin and the hardener is what you're looking for. There are two different commonly used materials- either the woven random fibermesh, or the material that feels more like a cloth, or mat.
If you've never used the stuff before, be ready for some strong chemical odors. As far as going about the repair, I would use a thin aluminum sheet, (like a pot pie dinner plate) to fashion a backer plate or mold, to keep the liquid from migrating all over- even a patch made out of duct tape could act as the backing to provide a basic shape to what you're trying to do.
You'll have probably 10-15 minutes of working time- depending on how much hardener you added to the resin, and how warm the conditions are when you're doing this. If you cant get it done in one batch, you can add another. After it all dries, I'd sand it relatively smooth, you'll have sanded thru some of the mesh- and then apply just a coat of prepped resin, so it can smooth the surface out and make a final sanding easier.
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