All this material came from the ReStore (moisture resistant drywall scraps, about equal to 2-3 sheets, which were $5 for the lot) and from a 'damaged' cart of DensShield (this stuff) from the big box. They wanted $11 (and I had a 10% off coupon to boot!) for the cart full of 12 sheets, so we took it immediately and worried about whether we could use it later.
Turns out that yes, you can use it as wallboard and it can be painted if you skim coat it with JC first. We were shocked to find that the existing stud walls in the cellar were
indeed built on 16" center, so the sheets screwed right on perfectly -
no long goofy cuts! It took seriously an hour or so to sheathe most of
the room (we're only doing the top half again). Ken did the skim today and we are priming - PRIMING - tomorrow night! The primer will really help to make the space look less, umm, Technicolor Dreamcoat? It's very Spring Festival of Pastels down there right now.