Hi Paul,
You would normally use sanding sealer as the base coat before adding varnish or paint. If you put sanding sealer over the top of the yacht varnish it could react badly with the varnish, especially if the sealer is cellulose based. In any case it won't be any use as you have already sealed the surface with the yacht varnish which isn't ideal as an undercoat for paint as it is intended as a final finish and is slightly flexible to allow for thermal movement in differing weather conditions. Others on here may have different ideas but I think your best bet would be to rub the yacht varnish down level with the wood and then apply an oil based primer/undercoat followed by oil or enamel paint for the final coats. Now that the wood has been impregnated with yacht varnish any type of acrylic finish is no longer a feasible option.
Colin