You should probably use a professional grade chemical if you plan on doing hard water stain removal, which is considered glass restoration. You should also be aware of "Tin Etch Haze". Most glass has a thin layer of tin on one of the surfaces; if you apply an acid, it etches the tin, thus ruining the window. You can detect which layer is coated with tin using a UV light. The UV light will case a gray hue on the tin side, whereas the plain "safe" side will reflect clearly. Look into SafeRestore made by EastCo Chemcials and another product from which im unsure about the manufactor, BioClean. Crystal Clear 500 (CC500) by Winsol is also a good product. Removing hard water stains is much more complicated then rubbing toilet bowl cleaner on the glass. Because of that, you can charge accordingly.