Earlier this month, I was working on a plywood project on which I wanted both clean laser cut lines (free of the smoke marks that often appear, especially on thicker plywood and MDF) and the ability to add color to the work without creating a separate silkscreen or stencil. I found that painter's tape or other low-tack paper masking tapes solve both problems. The process I used follows.
Laser-grade plywood or MDF
Sprayable clear wood sealer
Step 1: Apply painters tape
Apply painter's tape or other low-tack paper masking tape to both sides of the work piece for best results. If the work size is larger than the width of the tape, make sure each strip overlaps, but only by about 1/8".
The lower-tack the tape, the less likely it is to pull up the grain of the wood when removed.
Applying tape to both sides will prevent the appearance of smoke marks on both sides, but may not be necessary if only one side of the final piece will be visible.
Do not use plastic-backed tapes. Most will melt providing poor protection and possibly marking the piece, and vinyl tapes will emit chlorine fumes, which are poisonous and can damage the machine.
Step 2: Etch and cut
If not intending to apply color to the work, etch and cut the piece as you normally would, but increase the laser settings (power up, speed down, or a combination thereof) for the etching fields by approximately 25% to account for the additional thickness of the tape. It's worth running a few small tests to dial this in. More power may be needed to cleanly etch through the sections where the strips of tape overlap. Vector cutting lines should not need to be adjusted. You're done!
If you are intending to apply color to the work, still perform the cuts, but only etch the fields in which you do not wish to apply color.