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Hand-held finishing sanders use a quarter sheet of sandpaper. Each new sheet of sand paper has to be divided into four equal pieces. Using a straight edge to mark it and scissors or a utility knife to cut it will work--if you do not mind dulling the scissors and the knife in short order. Why not make a dedicated sandpaper cutter that leaves the edge on your scissors while making the sandpaper cutting easy? Here are the steps to construct and use this handy tool.

Attach a Thin Strip of Wood Flush to a 3/4-Inch Plywood Base
Attach a thin strip of wood flush along one edge of a piece of 3/4-inch plywood base that is approximately a 12 x 18-inch rectangle. Your dimensions can vary as long as it is big enough to accommodate the sheet of sandpaper. Make it the same length as the base and secure the strip with a pneumatic brad gun.

Attach a 1-Inch Strip to the First with a Piano Hinge
Attach a second strip of wood exactly 1-inch wide to the first strip (not to the base) with a piano hinge and small screws after butting their two edges together. We used a piano hinge, but any small hinges will serve. Attach the hinge so that the small strip will hinge back on the larger strip.

Fold a Full Sheet of Sandpaper Lengthwise to Measure
Fold a piece of sand paper in half lengthwise to determine how far away from the narrow hinged strip of wood to place the cutting blade.

Trace the Edge of the Folded Paper for the Blade Position
Lay the folder paper on the plywood base, butted against the hinged strip and trace the other edge with a pencil.

Lay a Hacksaw Blade on the Trace Line and Mark Screw Holes
Lay a new hacksaw blade along the traced line on the same side as the hinged strip and trace the holes at either end for screw holes. You can use an old blade for the cutter, but a new sharp blade is not very expensive and will cut better. Pre-drill the small screw holes and lay a small washer over each.

Insert the Screw Through a Washer, the Blade, and Another Washer
Insert the screw through a washer and through the hacksaw blade and then fit the screw into the washer/hole on the plywood base. Be sure to position the sharp edge of the blade away from the hinged strips. Tighten the screw in the plywood base. Repeat for second screw.

Extend the Hinge to Cut the Half-sheet
Hinge out the narrow strip and insert the long edge of a full sheet of sandpaper under the blade, butting it against the wooden strip. Tear upwards against the blade to cut the sandpaper lengthwise.

Fold the Hinge Back to Cut the Half-sheet into Quarters
Insert the narrow end of the half-sheet of sandpaper under the blade after folding back the narrow hinged strip. Pull up against the blade to cut it.