I often use 3/4 MDF for special window wraps in my custom homes. MDF is basically just "pith" that will turn into pus if allowed to get wet. I use an oil-based primer/sealer, especially on the cut edges to keep it from spongeing up moisture from mud/taping and any potential weeping of the windows. I thin it to about 50% to get maximum penetration and hit it twice. It really stops any moisture absorption and any finish coat goes right over it. I think Stewart's suggestion is valid. I'd almost be tempted to soak the plank ends in the sealer for a few days to really let it drink up, give them a week to dry, and then work the material.