What they use on cars its a completely different beast than what you SHOULD use on kayaks, either its soft stuff that doesnt bond well with epoxy or stuff that even if epoxy its in any case too rigid to be on an hull. If you go in a car paint store they will tell you that its perfect that they use it on sailboats, motorboats and this and that, dont let them fool you, you might as well use toothpaste.
If you want a soft filler use epoxy and microballoons, sands very easily. If youre concerned about softness then try to eliminate the problem at the source either using a top layer of the thinnest cloth you can find or, and this could be easier, try covering the wet resin with some "peelply" kind of material in a sort of outer mold fashion.

Or try tricking into sanding some friends that want to build their own kayak too, Huckleberry Finn style, lure them with some free beer and tell them its important practice. Do not use the "karate kid" line or it will be too obvious.

: As Im painting the kayak I am currently building I was thinking of using some
: kind of easily sandable filler instead of just filling the cloth with
: ordinary epoxy. Sort of what I see when watching the guys in American
: Hotrod (Discovery channel) fairing the body of a car.

: Any wise comments on that idea from you guys??
: Maybe recomendations on specific products.

: Gennie
: www.byg-kajak.dk