Commonly recommended chemicals for this cleaning task are sodium percarbonate, ammonia, or hydrogen peroxide, mixed with warm water.

Zee's product, Strike, has a sodium percarbonate base combined with quaternary ammonium chlorides. This might be an ideal product to try in this situation. It is, of course, a powder which can be applied straight or diluted with warm water to achieve the appropriate solution.

Another type of product often used for this application is one containing phosphoric and/or hydrochloric acid. Zee has many products that fall into this category -- Sparkle, 3225, Stay Bryte, to name few. Contact us for further recommendations.