Best sealer for the underside of soles .....
probably an oil based enamel. The advantage of sealing is the plywood wont swell and change dimensions when cold/cool when you do get a lot of condensation under the sole; plus, oil-based enamels are much easier to clean when removing the inevitable mildew that begins to form/grow on the undersides - Id consider to add a 'mildewcide' to the enamel - so you dont wind up with the typical/usual 'boat stink'. For ply you also have to seal the edges; and, since this 'shows', the best is usually what you use to coat the top surface. Its probably really best to 'seal' all the ply edges with solid thin bands of teak glued on with Resorcinol™ (not epoxy) for bombproof edge sealing, resorcinol™ is the 'glue' used in making 'marine' plywood. 'When' (not if) the sole begins to rot from water intrusion, its going to start at the side edges .... water following the 'grain' of the ply. Best sole coating is probably a product called "Ultimate Sole" which appears to be a copolymer of urethane and latex or some 'rubber' .... BEST foot traction especially when the sole is wet plus it seems to have good abrasion resistance (not found with varnish nor cetol, etc.). Just like when varnishing exterior surfaces I think most interior coatings should be applied THICK so that the water vapor permeability into the underlying wood is retarded - to prevent swelling, etc.- one or two coats just wont do the vapor barrier job. Id also avoid most 'topside' coatings such as Cetol, Spar varnish, etc. because they are too 'flexible' and therefore not HARD enough for 'foot traffic' and therefore are easily damaged by granules of sand, etc. that get carried in on shoes, etc. ..... for a sole I'd seek out a HARD BAR-TOP varnish (very difficult to find nowadays as most 'varnishes' are soft/flexible 'spar' varnishes) but the "Ultimate Sole" (my choice) seems to do well versus 'abrasion'.
