So I made a wedging table with a built-in cutting wire etc. The plaster is over 5 in thick and has set and dried for months. One huge problem though. I tried recycling thrown clay scraps and after it dried to the right point I started wedging and found that the clay started pulling up small pieces of the plaster surface, which obliously made the clay unusable, or at least unfireable. What can I do to fix the table with out pulling up All the plaster?