I removed several layers of wallpaper from an area with drywall. Between two layers of wallpaper, inexplicably, was a layer of Redgard (or a similar elastic waterproofing substance). Most of the Redgard came off, but thin strips were left on the drywall where the wallpaper seams had been.

What is the best way to remove this stuff? It won't peel off. I applied some OOPS solvent/cleaner, but I don't like to use it because of the fumes. I have had some luck with sanding, but this is time-consuming and runs the risk of damaging the drywall.