Epoxy resin will work excellent for this. (This may be what T4240Z is referring to.) And if you buy it at a marine store, you can get thickeners that will thicken it to the consistency of peanut butter to eliminate problems with runs. Just make sure both the bucket and fender are clean and bare metal/fiberglass (no paint) before you apply it. Use latex gloves, too, as the stuff is hard to get off of hair, clothes, fingers and anything else it touches. Also be aware that you must wait at least 24 hours for epoxy to properly set, and that it is much more time consumning to sand than bondo. Very tough stuff, though, impervious to everything up to about .50 caliber and should last the life of the car.