Although I use solid wood strip laminated foils I would finish a ply board the same. Having stripped previous varnish or paint and sanded the ply I have coated one coat of West System epoxy, followed with multiple coats of two pack polyurethane paint or varnish all applied with a gloss foam mini roller. Leaving to harden and rubbing down after about every three coats and possibly about twelve coats in total. Finish final coat to 1200 grit followed by paint cutting compound. This is for a high quality racing finish, you could be less fussy for a cruising finish.