OSB2 Cladding and Primer?

Hi,
New to forum.
Iv'e some really rotten window frames in the free-standing garage but have managed to make good the frame. For current financial reasons I have decided to board the opening with OSB. I have some OSB2 which isnt as good as OSB3 of course (waterproof etc). Once I have put the board over the opening I plan to use some UPVC Clad panels I have to cover the board. I still think I need to seal the OSB2 in case there is some moisture build up or slight water gathering over time. The question is as follows please:-
"What could I use to seal the board before I add the clad?. Could I use normal gloss paint (which I have) as I am working to a small budget OR do I need to prime the wood which would then not need any other paint covering?".
There may be other suggestions ref sealing that may be better (cheap please). Also should I put some mastic (or any other suggestions please) around the edge of board where it meets the brickwork?
Thanks...
M