FWIW, last year we installed a Kohler non skid tub. As most owners seem to note, it had gotten kind of grimey looking. Seeing some suggestions I tired this and it worked well. Got the surface wet and lightly scrubbed in a good dose of Bar Keepers Friend. Apparently there is some concern but supposedly its use is endorsed by Kohler. Then I covered the slurry for a few hours with a wet paper towel. With a light scrub with a nylon kitchen type brush, cleaned up very well, pretty quickly--like five or no more than ten minutes of scrubbing. Its probably cleaner than when I installed it. To originally protect the tub when tiling, I covered it with contact paper and a couple of old area carpets which worked well, but it still got some grime by the time I finished. With the BKF and a bit of scrubbing, looks better than new. There is some other product that costs $40 odd dollars that supposedly works well but seems like a lot of work and a lot of money. The BKF costs just a few bucks for a can.