Sealsatic works fine. It forms a thin skin where exposed to the atmosphere (seals) this can be painted over with primer and then antifouled and remains exceedingly sticky (elastic) where not exposed. You will find that it squeezes out a treat over the following weeks / months as your planking swells. The old salts used to use red lead putty below the waterline - linseed oil putty loaded with red lead powder - and white lead putty above the waterline.