Exterior deck repair

I have a deck made of marine grade plywood. I primed and painted deck with good quality exterior paint, or so I thought. Deck is about 8 years old and surface is starting to crack. Should I sand surface smooth before refinishing or will this weaken glue? Can anyone recommend a good exterior paint and primer to use. Can anyone offer a suggestion for a product to cover deck, i.e. vinyl or some kind of membrane?

Thank you,

joeman