A few weeks ago I found a set of hard fiberglass motorcycle saddlebags at a garage sale for $2, in fairly poor condition. I de-rusted the hardware and painted it flat black, patched up the cracks in the fiberglass and painted them to match the bike. I'm learning how to paint, and was still experimenting with the tops, so I didn't re-attach the safety chains...oops.
On my way home from work, I apparently did not properly latch one of the lids, and without a safety chain it fell off. I didn't notice immediately, and by the time I found the lid, it had been hit and the last 3 or so inches was broken off.
The two lids are identical, so I've been trying to use the intact lid as a pattern to repair the broken lid with fiberglass. The first attempts at the form were with expanding insulation foam and plastic wrap so I can remove it. I think the method could work, but the expansion makes it difficult to get an accurate form with a thin enough layer to cure in a reasonable time. Worse, the outer layer appears to be fully cured, but removing the foam from the lid revealed that the center was almost completely soft and liquid. A layer thin enough to cure would not duplicate the shape properly.
I gave up on the foam and made a form out of fiberglass. I greased the inside of the good lid, lined it with foil and added a layer of fiberglass and resin. I used popsicle sticks and binder clips to hold the fiberglass in the sharp corners of the lip. This form turned out well, so I riveted it to the broken lid. I was going to use the same combination of grease and foil to separate the new fiberglass from the form, but saw honey and figured it would be sticky enough but easier to clean. That part worked fairly well.
I need a couple more layers of fiberglass for strengh, then I'll bondo over the repair so I can smooth it out a lot easier, but I think it is going to work.