Another sealing question

We have a 3-year-old utility building with vertical walls made of Stimpson Plywood Siding (Dura Temp). It looks exactly like pine or cedar siding. In fact, we thought it was wood until today when we started to put up a trellis. The siding is made up on several wood plys with an outside ply of masonite with a grain pattern stamped into it. Is there anything you can seal this stuff with to make sure it doesn't take water?
Thanks! Suze
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