Typically, I would recommend a good quality oil primer for bare wood. If your paint on the surface is latex, I would stay latex. Zinsser 123 primer works well on exterior surfaces. You could also use Zinsser Peel Stop primer which works well to bond peeling paint. I typically recommend staying with the same brand of primer as your paint. As for the question about about one or two coats of primer, one is sufficient. I would use two coats of topcoat.

Uh Oh. I think the numb knuckles who built my deck used pressure reated lumber and then painted the rails. The painter I called to quote the job says if it is PT lumber the paint will never stick properly. Ugh. Last year the deck guys washed and stained (painted) white spots and those still hold. The remaining paint appears to be peeling down to the bare wood. Am I screwed?

Paint and stain are two extremely different products.

I recommend you call the Superdeck company.

1-800-825-5382