It's all plastic, IMO-both epoxy primer & body filler have two petro-based mixtures which chemically cure when mixed. Both stick like crazy to anything-I gotta chisel bondo off the dirty floor where it falls, and epoxy primer overspray sticks to anything/everything in the shop.
I've done both, and both methods seem to work. I do like the epoxy for those projects that linger-a buddy has a 51 epoxy primed as a finish, has held up very well parked outside and driven down dirt roads.
