Depends a lot on what the stain is, and not sure on powder. We try various approaches, rotating from one to another, and back again, and some times, nothing much works. If the glass is actually etched, then nothing will help, but you can try soaking in vinegar, and that some times works on mineral deposits, like from long time water contact. And we have also used some tub and shower cleaning products, one called The Works has helped us a couple of times, when vinegar failed, but again, if the glass is etched, nothing much will help. Acetone can help on some stains, but you can't use that is there is any hand painted decoration. Some of these products are very strong chemicals, so be sure to use proper protection for your hands, and plenty of ventilation. Acetone is flammable, so be careful, and keep it away from any flame source. Good luck!
