To keep sanpaper discs from curling up, make a simple jig to keep them flat. Cut out two pieces of 1/4-in. plywood that are slightly larger than the discs. Drill a 1/4-in. hole through the center of each of the pieces. Insert a bolt through one piece, lay the discs on it, and follow up with the second piece. Fasten the stack with a 1/4-in. bolt and wingnut.