There are 2 ways to do it. In this case, I made a casting “bath” with mdf and then lined it with release tape. This is a fancy word for parcel tape! I did this because one board was slightly twisted, so I wanted the resin to fill the space underneath. I then clamp the boards in, taking the twist out of the poor one. Then the whole thing is filled with 2 separate pours (I’m simplifying a bit).
If your boards are perfectly flat and parralell then you can get away with just a bottom board of tape lined mdf, or malamine or other suitable material. The boards are then clamped and I a put a small bead of silicon down to seal and prevent expensive resin leakage. You also need to dam the ends of the river as well.
For big tables, I cast them direct to the mdf and let them stick. This is because anything bigger than say 6’ is going to be a monster to sand, So it goes to a local CNC place to be flattened. Having it bonded to the mdf gives a perfectly flat surface for the cnc vacuum to draw against and holds it rock solid. Once the top is milled flat, it is flipped over and the mdf milled off.
