Mary,
I wouldn't use a damp cloth on paper/card to wipe off soot or residue....what does your white stock look like after you have done that? Use a medium to hard bristle art brush to brush it away....easier, cleaner and will not damage the cardstock.
Jit Patel
London UK
30w Trotec Speedmarker CL (Galvo) with 400mm & 250mm lens
80w Trotec Speedy 400 with Rotary, Vacuum Table, Cutting Lamellas, 2" & 1.5" lens, Pass through
Oki Printing Press
Abobe Creative Suite - Illustrator, Photoshop, InDesign.
Vector Magic