Porch Replacemenet

We need to replace rotten floorboards on our front porch. Have cut away the rotten bits and plan to lay exterior ply straight over the rest and seal, then paint with paving paint. The porch is covered but gets a lot of direct sunlight and rain coming in. Will the ply stand up to the exposure if it is sealed properly? What are our other options?

I would have thought exterior ply would have been fine, however another option would be cement sheet.

How do you plan to stop the paving paint cracking at the joins in teh sheet? It depends on the quality of teh subfloor I guess.