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'Tis a very good technique, used by boat makers world wide. Works well. Don't know why it isn't used often in autobody, but it isn't. If you wish to try your hand at it, fiberglassing will give you a much stronger repair then bondo will. I'll give you a quick rundown, and let you consider it. Polyester-resin:
This is the fiberglass jell stuff you find at K-mart and such. Cheap, strong, smelly, and kinda hard to work with. It turns into stiff jelly very quickly, and doesn't wet out well (as in wet the fiberglass cloth). It tends to lump up and make big thick repairs that require a lot of sanding. But it is dirt cheap. Epoxy:
This is the good stuff, available at boat repair stores. It's pretty expensive (about $60/gallon), but it works so nicely. Very liquid, wets well, long working times, quite strong. You can get it in different formulations to set up quicker or slower, and you can get all sorts of fillers to thicken it. This is the stuff used to make those beautifull wooden canoes. It is fantastic at automotive repairs. On the fiberglass itself Tiger hair:
This is just loose strands of fiberglass mixed into the resin. It's almost always the polyester-resin. Goes by the names of Tiger Hair, Kitty Hair, etc. I personally do not like the Bondo brand formulation,but others do. It's good for filling in compound curves, where cloth doesn't like to lay. I'd recommend it instead of bondo because of its strength and water resistence. Cloth:
This is just woven cloth made from fiberglass. Very strong along the lay of the fibers, less strong diagonally. Very usefull in doing large area repairs. Hard to wet out with polyester resin, but very easy to wet out with epoxy. Drives you nuts on big holes in metal because it simply sags. You've got to support it until it cures for this. Aluminum foil, waxed paper or plastic wrap all do a decent job of this. Cloth can be difficult to work with if the surface has compound curves or tight curves. The cloth tends not to want to lay flat then. Working with weights over it helps settle the cloth in, as does laying cloth on the bias. Mat:
A felt like mass of fiberglass strands. Used usually as a bulking and strengthening agent in building up boat hulls or decks that are to be walked on. Usefull for stuffing into areas of the hood before applying the resin. Can work nicely for making a patch on a rotted fender, if it is layed on the wheel side of the fender where it won't be so visible. I can babble on quite a bit more about fiberglassing if you'd like. I've built a kayak by hand this way, and have done some pretty extensive boat hull and deck repairs this way. I'm no expert, but I've learned quite a bit about the stuff.
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