Hi there
First may I say, Congratulations on a superb build ZZ56
May I make one suggestion? You could find it is best to give the hull one coating of polyester resin with the correct amount of hardener mixed first allowing this coating to dry. This coating of resin will soak in to all the gaps between the planks and seal them for you. After this coating has dried, if you lightly sand down then look for any hollows in the planking, this then can be filled with car body filler and then smoothed flush.
When you are happy with the finish, proceed in giving the hull another coat of resin/hardener mix - with a layer of tissue matting added. When this has all dried, you could then give a final coating of pure resin with hardener mixed. This will give you a workable surface to produce a good finish on the exterior of your hull through sanding.
The other thing is as well, with giving the hull a coating of resin/hardener mix first off, it will also harden the balsa blocks at the stern thus preventing too much being sanded off these easily.
Hope this helps my friend - carry on with your build - very enjoyable read
Aye
john