Typically you proof the 2nd floor with a torch down or membrane and or something similar then use duck boards, but it's dependent on the system as well as there are light weight concretes, vinyl, T&G and a myriad of other finishes.

I don't think it's possible to go back and water proof it totally... what do you have up there?

http://www.altereagle.com/

http://decks-ca.com/

http://kingofcrown.com/

http://installcrown.com/

Alter Eagle Construction & Design

3/4 Inch treated Plywood. Can you recommend one of the various products to use on top of the paint. Nobody in our home stores have the answer for me.

You can see if a Vinyl installer can place a membrane over the ply but I believe the paint will be an issue now.

There is lightweight concrete finish such as the one I used on the "greenvalley" project. That can get expensive as would tile.

Then there is system where you use tape & paint but I find it fails within a year or two without proper maintenance.

Lightweight concrete would be the best. Look for decking in the yellow pages for an installer in your area.

http://www.altereagle.com/

http://decks-ca.com/

http://kingofcrown.com/

http://installcrown.com/

Alter Eagle Construction & Design