"Gumption" Paste Cleanser for Shower

I had some very intractable marks on the window that forms part of the shower in the bathroom (it's very private here!). These seemed to be marks and residue left by water evaporating to leave the calcium and magnesium salts behind, rather than soap scum from the shower.

Newer deposits on Perspex came off easily with white vinegar, but marks that had been on the glass window for several years would not budge! I tried: white vinegar, spray on window and glass cleaner (probably contains some ammonia), tried scrubbing with a nylon scourer, "Tarn Off" silver cleaner (contains thiourea and sulfamic acid),caustic dishwasher machine powder, "JIF" cream cleanser, no-scratch ceramic cooktop scourer with aluminium threads, toothpaste (two types), oven cleaner (with sodium hydroxide), hydrochloric acid (dangerous stuff, do not use unless you have researched what you are doing and take all necessary precautions) and finally (drum roll); "Gumption" multi-purpose paste cleanser made by Clorox.

While the vinegar/acid and glass cleaner might have made some slight improvement, the ONLY one of these that really worked was the Gumption. Applied with the no-scratch glass scourer and a little bit of scrubbing, the windows have come up brilliantly. At last!