whats the hull made out of? You could use car body filler and sand the heck out of it. If it's polyester you can mix up a paste from resin and pure talc (or microballoons) or do the body filler thing.
just mix 'em up and mold it on- you can put a dab on, then mail-tape over it- the mix wont stick to the tape, so when it's dry you just pull the tape off and it's perfectly smooth.