JB weld is an epoxy based adhesive with fillers for adhesion and resistance to abrasion (very likely finely powdered aluminum, talc, and lord knows what else). It should actually hold up quite well as far as abrasion resistance goes.

If you want to fill with epoxy, I would make a mixture of any good laminating epoxy (West, MAS, etc.) with aluminum powder (abrasion and UV resistance), and cabosil. The cabosil will make it very thick and almost impossible to sand...this is just the long way around to make your own JB Weld (but you can make it as thick/thin as you need).