Hook-and-loop backed sanding sheets — the part of the sheet stock that attaches to a sander pad instead of clamping. Shapes cover 3-1/8 and 3-1/2 in. triangles for corner and detail sanders, MegaMouse rectangles, and 3-1/8 x 5-1/4 in. sheets with 14-hole dust extraction drilled for the Festool RTS 400. Hole pattern matters as much as size: if the holes do not line up with the pad, dust extraction stops working.
