the beeman,
Yes, you do need to worry about getting the B/C plywood painted with Kilz very fast. The plywood sheet does not bend upwards on the ends just because of humidity. It does this even in a climate -controlled environment, like the Big Box Stores.
It bends because the top and bottom of the sheet do not dry at the same rate. The top dries more quickly and contracts. As some will tell you, this can happen just overnight. And, if you leave it out in the sun, it will bend up in two hours.
If you can't promptly paint it, then lay it down flat on a clean dry surface, out of the sun, and put some good weights along the entire length of the butt edges. I use tool boxes or boxes of books etc. This will keep it flat. I have kept it flat like this for a week or two with no problems. (If your floor is damp, though, forget it.).
Painting it is really super easy. Get a cheap nylon 4 inch paint brush and just go to work. Takes about 5 minutes to paint a side, and about 45 minutes for it to dry. I put an old blanket on the floor, put a few 2x4s flat on top of it, lay the plywood on the 2x4s to keep it off the blanket, and paint the first side and the edges all at once.
My understanding is that cabinet grade and birch plywood do not have this issue, though I have never used them.
Mannyrock
