Dumond Chemicals makes a paste stripper that can safely remove many layers of paint in one application. The secret lies in the paper that is applied to the glass after spreading the paste. The paint is lifted and transferred to the paper in one simple step. Read package directions carefully, though, because the company makes several types of paint remover and not all of them are safe for use on glass.
Citristrip, made by Kleen-Strip, is a thick paint stripper with a pleasant orange scent. It contains no methylene chloride, which produces volatile fumes, so you can use it safely indoors. Several other "green" paint strippers are available, but not all are labeled safe for use on glass.
The Parks Corporation makes a variety of stripping products in liquid or gel form. The No Drip Strip product is a paste made for cleaning vertical surfaces without dripping. Visit a home improvement store to view a wide variety of stripping products. Always read the labels to ensure that the product is safe for glass. Apply stripping products to an inconspicuous area first. Some products may etch glass or damage lead in stained glass windows.
Professional painters usually remove paint from glass with a window scraper or razor blade. Spray the window with glass cleaner first to create a slick surface and reduce the risk of scratching the glass. Gently scrape the paint off to remove it. Nail polish remover or citrus-based cleaners can also be used to remove paint, but try these products on a small area first.