I have just acquired 84" rails for my tablesaw and constructed a new outfeed table that spans the whole width. Now I want to build a filler table at the side of the table to continue cast iron top surface between the rails (approx 27" x 36"). I have some 3/4" MDF in the shop, but it appears to have some warp and twist (about 1/4" corner to opposite corner). I have enough MDF to make the top two layers thick and have clearance below to add deeper wood supports to help keep the top flat. When I tried dry fitting the two top panels with clamps I still show a little twist. Before I try laminating the top pieces together I want to be sure I am not missing some simple tricks to flatten the surfaces permanently. Can the warp in these panels be permanently removed or is the material beyond hope in creating a surface flat enough for a tablesaw table?
Rather than laminating two pieces of MDF together which is not guaranteed to remove the warp you see I recommend you build a torsion-box style lattice structure out of straight stock and skin it on each side with your MDF. The lattice structure is incredibly rigid and because you are building it with solid wood you can mill it to ensure consistency and flatness. Once you fasten the MDF on top and bottom of it (glue should be sufficient) it should overcome your warping.
I am assuming you are trying to make use of what you have in the shop, otherwise I maintain my recommendation but would use thinner MDF for the skin.
All I do whenever I have a warped MDF board is put it down on a flat surface and weigh it down and leave it. For me that works every time.