After many tests using powders, pastes, gels, scrub pads, soft scrub, bar keeper’s choice, acids, razor blades, and steel wool, I have found a method that works for me and seems to be consistent with results.
Tools:
3″ Rotary Polisher with variable speed
3″ glass felt pad
Bio-Clean Hard Water Stain Remover
Soft Scrub Cleanser
Glass Micro fiber towels
I do like to clean the glass with a soft scrub and white nylon pad if there is a lot of soap scum on the surface first. Using a 3″ glass felt pad with a car detailing rotary polisher that has a variable speed works for me as it concentrates on very small areas that scrubs effectively. One trick is to attach a small led light to the polisher so you can see when the hard water spots are being removed in real time.
It still takes about 1-2 hours to clean a standard size glass slider and is a lot of work, but the results are very satisfying.