ALL my balsa float are sealed with Cellulose Dope and have been for donkeys years. I always make the effort to seal, lightly rub down and seal again, before another minimal rub down to remove the shine and to act as a bond for the white undercoat. Might sound belt and braces but nothing worse than a float failing because of poor preparation.
NOT to say this is the only way and other methods might be available, but it works for me.
I also use Morrells Severe varnish x 20 % gloss OVER the painted finish which some might feel is OTT (some question the 'gloss' bit) but my floats last well despite heavy use AND they catch lots of fish, always the final test, imho!
ps The Dope can easily be applied (slightly thinned if reqd) with a finger, BUT this needs removing from the skin asap - and I mean ASAP!!