Finish on all four sides of any decking project, covered or not, is a bad idea for a lot of reasons.

Ventalation and fastening are much more important. J.

If the floor is to be painted, follow the label instructions of the paint you will use for its priming requirements; the porch paint I've been using (Ben Moore Alkyd) is so-called self-priming, and works best as its own first coat.
With PT yellow pine T&G, to get a decent finish you will need to have the floor sanded before painting to get it flat. Un-sanded, there will be proud edges that will get the paint worn off them in short order. The sanding (to 80 grit, but no further than 100 grit) will roughen up the summer wood parts of the yellow pine grain so paint will stick; peeling is the worst characteristic of YP as an exterior wood.
If I were using oil paint, I'd thin the first coat with a little turpentine, it really seems to help paint stick to YP.
Casey

Take a look at CorrectPorch T&G Boards - Composite

The Porch Guy