The POR15 people sell some excellent filler material. Much easier to work with than the JB-weld epoxy. I use the JB-weld a lot but as a binding epoxy and for unseen areas. Danged stuff is too much work to sand and form.

The POR15 Epoxy Putty goes on a bit tougher due to its consistency but you can shape it with wet fingers, sets in 20 minutes and sands like a dream.

They've got some videos on their web site