This is a great question as I pondered this myself when fitting the floor ply.
Tigerseal'd treated buttons to the floor. Then used 9mm birch ply for the floor itself. I sealed the ply all over using a product widely used in the furniture industry, an Acid Catalyst sealer/lacquer. This is a 2-part product, i.e. you have to mix an activator in correct proportion. Fantastic stuff, fittable dry in an hour, good water/moisture/chemical resistance. Great for mdf as it make the surface & cut edges go rock hard, sands back almost like a glass coating.
Not that expensive, around £60 for 5 liters, Roller it on. Manufacturers? ISF Coatings, Morrells, Fiddes. Intercoat, just Google for "acid catalyst sealer (or lacquer)"
p.s. its a heavy solvent based product, so be prepared, nitrile gloves, ventilation. Will eat the foam on a roller very quickly so have spares.