Masonite, the brand name of Masonite International, resembles extra-firm cardboard. This utility material, used for such items as cabinet backings and underlayment for vinyl floors, was invented by William H. Mason in 1924. It's constructed by steaming wood chips to form long fibers, and then pressing these fibers into 4- by 8-foot sheets. You can cut Masonite with any saw that you use for wood, but it's even more prone to tearout than plywood. You can pursue a number of strategies to reduce chipping and edge damage, and produce a straight, clean edge every time.
Make straight cuts with a circular saw or a table saw. To get the best results, cut with a steel-tipped blade with 80 or more teeth.
Mark the cut line on the back of the sheet, using a pencil, and place the sheet face down when cutting with a circular saw. Suspend the waste side of the sheet over the edge of the cutting surface and clamp the sheet to the surface with C-clamps. The clamps reduce chatter while cutting and prevent the saw from wandering.
Clamp a straightedge to the work surface to guide the saw and guarantee a straight cut. Arrange the straightedge so that it's flush against the foot of the saw when the opposite side of the blade is on the cut line.
Move the saw slowly and steadily forward past the end of the cut, and let the off-cut fall freely. If the off-cut is more than 2 feet wide, you may want to get a helper to support it so it doesn't get damaged when it falls.
Feed a sheet of Masonite through a table saw face up to minimize chipping and tearout. The saw does the most damage when it exits the cut, and if the sheet is face up, the exit point will be on the back of the sheet.
Cut curves with a jigsaw or reciprocating saw and a multi-purpose cutting blade. Do not use a metal-cutting blade -- Masonite is fibrous, and the fibers will quickly clog the blade.