Painting - pva instead of primer?

I'm trying to finish my stair case that was built with 2x4 framing, with some drywall along the bottom corner. I've used pva on the drywall. Can I also use it on the wood, instead of primer? I live in Japan, and have checked into buying primer paint. Options are: spray can (would take quite a few) or buying a huge drum of paint from a store that usually deals with professionals. The 2x4's have a lot of knots, so I'd like to come up with as smooth of a finish as possible. Will appreciate any advice/thoughts.

PVA is good for bare drywall, but I wouldn't recommend it on wood. Try to find some BIN shellac-basec primer for the knotty wood. If not, then any alkyd or oil penetrating primer will work.

Sorry, what's BIN shellac? I've heard of shellac before but never BIN.
Again today I did some shopping, and stores do not seem to stock ANY wood primer other than in small spray cans. I will look for shellac, or alkyd or oil penetrating paint.
Also, will the 2x4 frame ever look smooth and nice (granted, nice is a relative term). I imagine there is a limit as to how beautiful it will ever look. We went with the 2x4 staircase railing because the housing company considered the railing as an afterthought, and we only had a limited budget by the time it was being built. It's strong enough, but scores low marks on beauty right now.

I noticed that I still have a can of Varathane that was bought when our other paints were bought from Canada. Is is possible to use Varathane as a primer, before painting the wooden staircase railing?