That can be tricky. If you only have a little bit of glue or paint on your glass, you may be able to get it off just by rubbing with a soft cloth. If you have any more, you may have to use more drastic measures like using a VERY FNE rubbing compound such as white toothpaste. You can also use a superfine polishing kit that many accessory manufacturers offer. If you wind up with a slight fog on the glass, you can usually eliminate it by using model wax, clear enamel or liquid acrylic floor wax. If the glue is embedded too deeply, your only choice is to replace the glass.