No underlayment needed. Its easy to install and comes out great. I sanded rough areas, filled holes, primed and I painted with a deck paint
. The deck paint
wasn't necessary, but I thought it would provide some additional protection for the subfloor. That's just me though. I usually do things for the "what if" situations. The nice thing about priming is that it really gives you a clean surface to start with. I don't think any amt of cleaning these old floors would provide the same "clean" surface surface.