Paint is not a good thing when it comes to covering a PT deck outside unless, of course, you are prepared to do exactly what is required periodically: sanding and refinishing.
The old paint must come off and the best way to do that is mechanically, not chemically as in paint strippers. The wood will absorb the stripper and you'll have one heck of a time putting a new coat on top of it. Nope, sand it off with a rented orbital sander and refinish it with the finish of your choice.
The best choice there would be a stain meant for wood. But again stay away from solid colours that look like a paint, these products have limited lifetimes, unless you are prepared to redo them.