From the MAKE Flickr pool
Macetech shares details on removing PCB resist with a laser cutter -
It’s easy to remove most of the residue using water and a paper towel. I used to use alcohol because it removed the residue almost instantly, but I found that it also eroded the paint mask too much. Water alone doesn’t get enough of the residue off, so you need a very mild abrasive polish. The best I’ve found for the purpose is ordinary toothpaste, although I forgot to bring it to Techshop and used some plastic polish instead. This also works fine, but it has a wax which needs to be removed with a grease-cutting soap before you attempt to etch the copper.