If the doors do not 'touch' the ground the frame will last 15yrs at least. If it is Genuine Marine ply i.e. BS1088 stamped, it only needs sealing to last. It all depends what finish you want. If you want paint, then prime, undercoat and weathershield gloss is the way. If you want wood, then Sadolin Classic will be ideal for the ply, Sadolin extra for the frame.