I tried Bondo. Worked well but eventually it came out.
Seems as if the best way to go is to use layers of fiberglass that is not woven but looks like matting. I got it from a friend and not sure where he got it, but I suspect that an auto repair shop could point you in the right direction.
The idea is to layer it on and roll fiberglass resin in till it is just soaked. The resin is only there to hold the matt in place. The resin doesn't provide strength. The glass matting or cloth provides the strength. Use carbon fiber if you can find some. If you use carbon fiber, be careful when sanding, as it is tougher than nails and will stick into you. So put only enough resin in to wet it, then squeeze out the excess. A roller works well. You can use a paint roller cut in two or three sections.
As Flumpmaster mentioned if you put Saran wrap or some kind of plastic over as it hardens it will come out smooth. Depending on where the repair is you'll have a smooth beautiful repair or a smooth ugly mess.
Then sand off the excess and fill in surface problems with something like Bondo. Trouble with Bondo is that it is dark and will display your handiwork so you have to coat it with gel coat. Too bad bondo didn't come in white.
Where I sail is rocky so white "Bondo" would be a good thing as my expression is that it is inevitable as death, taxes and dings...