I masked off the area needing the color fill (I'm engraving Rowmark material), using a paper mask. When I laser engraved it, there is residue left from the mask; the residue keeps the paint from adhering to the plastic. I wonder why it leaves a residue??? Is there another type of masking that would be better suited for this? I did a search and couldn't find an answer to this particular problem. Any and all advice is appreciated. Thanks so much!