When cleaning a limestone shower you can not use any type of acidic cleaning products. You also have to be careful even when using professional alkaline tile cleaning products on limestone. Improperly applying even professional cleaning products can stain or damage the finish of limestone. Limestone is also an extremely soft stone and any type of abrasive scrub pads or scrub brushes can easily scratch or dull the the finish of the limestone.
A professional level soap scum remover like MB-3 is the best way to remove soap scum in a limestone shower. MB-3 is a professional non acidic soap scum remover that will effectively remove soap scum without damaging the finish of the limestone. MB-3 also works to clean off hard water deposits on the limestone tiles.
When cleaning limestone you should always properly dilute your chemical cleaning products with water before applying them to the limestone. It is also important to water down and get the limestone wet before actually applying any type of cleaning solution to limestone. MB-3 should be diluted 2 parts warm water with one part MB-3.