Prime all six sides, apply one finish coat to at least the bottom before installation. Install and either apply base and top coat or just the second top coat to the top side. I would suggest oil base primer (or as close as you can get today) and an acrylic porch and floor top coat.

Fine and dandy........

BUT-

There are VERY FEW primers made for HORIZONTAL/FOOT-TRAFFIC use!!!
Cabot has one for underneath SOLID, dark-color STAINS.
MAD-DOG has a deck-primer too.

Most P& F solid PAINTS are "self-priming" on bare, sanded wood.
* FIRST coat needs to be slightly thinned with water to aid penetration.
* 2nd coat goes on UN-diluted.
* Yes...doing 1 coat of slightly thinned P&F on all sides is fine. 2nd-coat only the tops/sides.

Faron

OK, so I'm going to get Ben Moore Porch & Floor paint.
How best to apply to a large quantity if T&G paint to this wood? Brush? Roller? Dip?
And I've never thinned paint. How much water to paint?
Thanks!
Sarah