Hello Jan. IMHO, you may look for an epoxy type of filler. As you intend to sail this model, the hull must be waterproof. Epoxy should be your best bet. I didn't work with epoxy filler, maybe others can chime in?I’m now trying to find a good wood filler to eliminate and fill in some the voids. I’ve found Elmers wood filler hard to work with. I don’t know if I had a bad batch, but I couldn’t apply it smoothly, it tended to roll and ball up as I applied it.
What do others use?