The Stovebolt Site Forums The Shop Area Interiors Waterproofing cardboard panels

We always use Varathane brand of polyurethane on fiberboard and especially mdf. It doesn't prevent paint later and makes the stuff last much better. The first coat soaks in but the second one doesn't. Usually two coats are enough but if you're in a really humid area, 3 is better. Let all coats dry for at least 72 hours and preferably longer. Once that stuff soaks in the top surface skins over and makes the drying time a lot longer. Just be certain to get the edges very well coated. You can't really seal those too well. Otherwise humidity will get in and those panels will still warp even though the front and back is sealed. If you've ever seen a countertop made of particleboard which wasn't sealed you know what I'm talking about.