Hi Jeremy
Sorry for delay in answering but I wanted to give some of the other members the chance to answer this question.
As you are painting the hull rather than varnishing you could use a 2 pack car body filler such as isopon, which is available at places like Halfords, this filler has the merit of being very workable when set and produces a very smooth finish when rubbed down with wet & dry (1200 grit) it will accept most paint finishes and is waterproof.
Before applying the filler make sure that the timber is 100% dry and free from dust.
A word of warning would be to check the compatability of the paint finish with the filler by using a scrap piece of wood with some smoothed filler on it, apply the paint as you would on the model and check for problems such as blistering.
Hope this helps and good luck with the build.
Paul