depends on what model your building, and your method of finishing, if your going to monocoat(heatshrink covering) it then some wood filler works well for smoothing it so the area isn't so obvious, Minwax wood filler is one I use allot, Elmers also makes one, or just some light weight spackle, I would recomend sealing the area if using spackle before you cover it since spackle doesn't provide a super solid surface for the covering to stick to.

if your glassing the model( using light weight fiberglass) then fill it after words with a bondo spot putty or simmilar type filler, Evercoat makes a light weight filler that sands better than a bondo type and is lighter weight.u