(I speak for myself ...) I'm not a big fan of painting a floor. The previous owners painter our 3/4" plywood kitchen floor because, well ..., they ran out of $$$ to finish the kitchen! At that time, we bought the house from them and installed sheet vinyl.
ANyways, if you're wanting to paint the floor, first consider what your walls might look like if you painted them. The floor will wear MOST where you do your walking. So, you'll see 'trails' down the floor over time.
Interior or Exterior grade plywood is fine. You don't do anything to the seams in case the floor 'moves' on you from expansion and contraction (unless that is, your floor is already installed with sheathing that's cross-grained in layers ...) Use a latex-based Primer/Sealer. As for a topcoat, visit your local Paint Retailer (found in the YELLOW PAGES under PAINT - RETAIL) and ask them for options.
My best to ya and hope this helps.
Jay J -Moderator
PS: DO tell them if you have interior or exterior grade plywood. I'm not sure if it makes a difference when it comes to painting it. (Interior grade is much 'nicer' looking then exterior. It has a 'finished' side to it.) Remember, if you're installing more sheathing, you may have to cut some doors, and some of your 'transition' doorways into other rooms may be 'higher' than they are now. IF you can (or have to), rip up the top-most layer if you can and replace it. THat way, you dont' have to worry about doors and cabinet heights and such. Have you considered other flooring options, like sheet vinyl or tiles???