I am replacing the board that goes underneath a sink’s kitchen cabinet with a 1/2″ OSB panel; it lies flat.  (Don’t ask how we got there — what a mess!).  I plan to put quarter-round around the periphery and add caulk to try and prevent water from getting into the edges.

I have a sheet of 1/2″ OSB (Rated “Exposure 1”) and some Minwax Helmsman Spar Urethane (Water Soluble).  Should I consider putting plastic wood on the edges to help seal the OSB?  Would adding solid-wood edging to the edges of the OSB, gluing with Gorilla Glue help sealing the OSB?  I understand that I need to get a wax coating off the smooth surface before finishing — my sander should do that.

If I put 4 coats of this finish on, would the resulting panel resist standing water should the faucets or disposal leak again?  I would doubly apply finish to the edges.  Would oil-based Spar varnish/polyurethane be the better finish choice?  I don’t know how to thin the water-based stuff, but, if advised to use oil-based, I would thin the oil-based finish’s first few coats.

Am I OK with this approach?  What would you recommend to waterproof the OSB? What alternatives would you suggest?  (Min $$$)