Denatured alcohol or acetone will work in the most difficult areas, but be sure to do a spot test beforehand. Sometimes, you may run into paint on glass that has been adhered too strongly to be scraped off with just a razor blade. If this is the case, then you should try to soften the paint. This can be done with hot water, mineral spirits, denatured alcohol, acetone, lacquer thinner or even Goof Off, depending on the paint.
I usually follow this order and try the least harmful product first and can only reach the most aggressive chemicals if necessary. After loosening the paint, I scraped it off with a razor and a Windex. Apply a cloth, scouring pad or steel wool with hydrogen peroxide and cover the surface with liquid. Rub the surface topically to allow hydrogen peroxide to begin to break down the paint composition.
You can leave the surface to soak for an hour. Afterwards, the paint should be looser and much easier to remove. If necessary, add another coat of hydrogen peroxide to help loosen the toughest layers of paint.