It does what it claims.

I tried Zep Commercial Power Tub & Tile Cleaner. It is the first product that REALLY does what it claims. My kitchen sink is double cast iron/ porcelain model that is about 70 years old. It did some stains that would just not come out even with bleach. My daughter told me about this Zep product. Not cheap, about $7.50 for a Quart bottle. READ and follow the label and warnings! Very good stuff.
Mike