I have used Zep shower cleaner (generic Kaboom I guess) combined with cermabryte (designed as a glass cooktop cleaner) to attack hard water from calcium deposits on windows. I usually use a rag attached to the bottom of a corner sander to scrub the cermabryte into the surface after it is doused with the Zep.
I wonder if this is what @anon89336055 uses it for with the WFP.