Need some finishing help

I have a large wooden box on my back deck that I built to house cushions and the like. The top is in two sections made out of plywood about 3' x4' each, both attached with a piano hinge so they act as doors. I had finished them with a number of coats of polyurethane, but it keeps peeling off during the winter. Any suggestion on what I can finish this with that will provide protection and that will stay affective? Thanks.

Marine grade spar finish. Heat and cold resistant, also filters UV light.

Thanks Pierre. If I can find it I'll pick some up.

or...Thompson's Water Seal.
Poly sucks for outdoor use.
You are probably safe with most of the products that are formulated for decks.

Duh, why didn't I think of that? I even have some sitting in my closet. Thanks Mark.

Stay away from Thompsons Water Seal. It is nothing more than paraffin and mineral spirits. Sand the surface to get rid of as much of the polyurethane as you can then coat with spar varnish. Polyurethane is to rigid and spar varnish has tung oil added, along with UV inhibitors, to make it more elastic with changes in temperature and humidity and suitable for outdoor use.

Deck finishes will work but unbtil you get rid of the polyurethane, nothing will penetrate as needed.

I figured that the poly had to go before I applied anything. This is great info, thanks.