Pastel surfaces often need to be trimmed to fit framing requirements or to improve the overall composition of the painting. Once pastel has been applied, this becomes a more delicate matter and requires added precautions so as not to alter the appearance of the painting.
 
Useful items to have on hand are: glassine protective paper, X-Acto utility knife with fresh blades, heavy-duty utility box cutter with fresh blades (for extremely rigid surfaces), metal straight edge (a metal carpenters square works well), cutting surface, drafting tape, and an emery file board. In place of a metal straight edge, a mat cutter can be used.
 
Start by placing a sheet of protective glassine paper over the painting, taping it to the backside or extra boarder of the surface. The glassine protects the surface of the painting, allowing the straightedge or mat cutter to be placed on the painting for the cut. By tacking it in place, you can avoid slippage and potential smudging. Make sure the surface on which you're cutting is skid proof and won't be damaged by the penetration of the blade. Cutting surfaces are available at most art supply, quilting or craft stores. Keep the height of the table low enough that you can apply a moderate amount of pressure when leaning over the piece, helping to keep the painting and straightedge in place.

Mark the placement of the cut lightly with a pencil, then gently place the straightedge over the painting at the desired marks and score the painting with the utility knife. It's best to start with a weak incision, creating a track for subsequent cuts. If a mat cutter is being used, follow the same procedure, placing glassine over the painting and then carefully placing the painting under the cutter edge at the desired point. In my experience, it's been easier to use a utility knife with a mat cutter than the mat-cutting attachment, which often cuts a distance from the metal edge, requiring compensation. Many pastel papers are abrasive and, after a few passes, the utility knife blade will be dulled and need replacement.
 
Different surfaces need different care. Thin papers are the easiest, often requiring a single pass of the blade. Mounted papers may require multiple cuts to penetrate the mounting board. Rigid wood composite panels, like Ampersand Pastelbord, or homemade grit surfaces are the most difficult. It's easier to cut them on a table saw in advance of painting. Once pastel has been applied, these surfaces require a good strong hand, a heavy-duty utility knife and patience. Score the board with a few passes of the knife, then place the board over a straight table edge at the scored point and snap it down. If everything goes well, the board will snap along the scored cut and only minor sanding with an emery board will be required to clean the edge.
 
Practice on a few failed paintings before trimming your masterpieces. With preparation and calm resolve, everything will come out fine, and your paintings will have the outer dimensions you require.
 
Pictured here: A pastel painting in need of trimming, surrounded by a cutting surface, glassine, X-Acto knife, utility knife, drafting tape nd metal carpenters square.