That process uses a proprietary epson grind and works pretty well. The system they use does not work with ordinary Epsom I'm told.

Have you tried muriatic acid instead?

If I get calcium deposits, I use 4 parts water to which I add 1 part muriatic acid - usually in a regular plastic garden sprayer bottle. Wearing eye protection and rubber gloves, I spray it on the deposits. The mixture bubbles. Using a poly scrub brush, I brush the area until the deposits are gone - may take a number of repetitions of spray-and-scrub to get them off.

Once gone, I rinse the entire area with pool water. I dump any excess mixture into the pool as well, as it is what I use to lower pH anyway. Rinse out your spray bottle as well.