3 inch Hookit™ back up pad (Clean Sanding) is constructed of foam material. The backup pad itself has a 5/16-24 internal thread, and the kit includes five adaptors that convert it to 1/4-20 external threads, 1/4-20 internal threads, 1/4-28 external threads, 5/16-24 external threads, or 6mm-1.0 external threads.