I use MDO as a base which is much more stable than any other plywood that I have worked with. I put corian on top of that with the vacuum grid (4 zones) carved into it. The spoilboard tops off the system but it is doweled not glued. It stays quite stable for weeks at a time but if I am carving tight tolerances I dress the table fresh before carving. One thing to watch when dressing the spoilboard is the amount of vacuum you pull on wide open spoilboard as apposed to carving a sheet that has all of or most of the spoilboard covered. I cover as much spoilboard as practical as I dress it to keep the vac as high as I can.