Good ol' Pactra dope works well as a sealer and film coverings stick to it and WON'T come off.
Water Based PolyUrethane works well, too. Before you seal, Bondo or lightweight spackle fills the cracks, and the sealant seals the spackle so it won't get gooshy.

If you glass the floats, use your fingers the smear a thin but visible coat of spackle over the glass cloth, then take it down to the glass cloth with a fine sanding block.

In olden days we used to make a paste of Talcum powder and clear dope as a sealer/ filler, then sand that down slick before putting on the final gloss coat.