Try sign vinyl from US Cutter or such. it comes in many sized roles and is easy to apply and remove. I use it at my middle school with the kids and as long as you do not apply paint heavy you might not have to seal it! You can also spray paint the surface before you carve and you will get the surface one color and the grooves another.
http://www.glenspeymillworks.com Techno LC4896 - 2.2Kw Water Cooled Spindle | Moving Table Mill from Omis 3 CMM, 500Lb granite base | Epilog Legend 32 Laser Engraver