Would gesso work?? Mel (The Terrain Tutor on YouTube) suggested using watery wall filler to effectively seal both sides of MDF base plates.Are you using MDF for the base? If so, what did you use as a sealant? I used MDF on my terrain tiles project, and ended up getting a small amount of warping. I didn't seal mine, but subsequently heard that using some kind of oil-based primer is the ticket (water-based primers on MDF supposedly can cause warping). I ended up experimenting with the oil-based primer on a large sheet of MDF just to see what would happen, and it didn't warp. The rattle can version of oil-based primer is kind of pricey in my area though.