I say,
get a 3’ long flat sanding block (make one)
sand that bitch
check it with a square as you go
go forth and conquer!!!
Well if you don’t have any gaps I’d say throw in the stringer. Your halfway done.
alright… but how do i do a balsa skin?
Its your first shape, right? I say just glass it and surf it till it dies. Im pretty sure you didnt shape the perfect board on your first try, nor will the glassing be exceptional, so just use it as a learning tool for the next, and next and next and so on…
since youve it split you might as well add a stringer,i use 8mm or so marine ply
but its a case of whatever works for you.
to fill in any gaps you could try getting ceramic microbubbles,its a powder
you mix with resin to make a filler