I have four stainless steel fermenters. Three are made by SS Brewing Technologies, and have a conical bottom with a valve, dip tube, and thermowell. The last is by Chapman and is a cylindrical shape with a tap near the bottom. They are all good products, but I'm partial to the SS Brewing Technologies fermenters for their conical bottoms (to capture trub and yeast), stackability, and thermowells.
Although I purchased it (supposedly) new, one of my SS Brewing Technologies fermenters arrived with a rough white substance on the inside. I suspected that this was beerstone, and was hesitant to use the fermenter for that reason. Beerstone can create pockets for bacteria to thrive in, and it can be difficult or impossible to sanitize that surface well enough to prevent infection. I work too hard on my brews to chance an avoidable infection.
Recently, I started reviewing how other home brewers were removing beerstone from their equipment. The recommended solution is a two-stage cleaning with extremely hot water. The first stage uses a phosphoric/nitric acid mixture. The second uses an alkaline cleaner. After using the alkaline cleaner, a very thorough rinse is recommended.
Digging around online, finding the recommended acids was difficult if not impossible due to concerns over their hazardous nature. My stand-by cleaner, PBW, is an alkaline one so I figured it might do the trick if I could find the acid mix. Star San contains one of the acids, but not both.
I continued to research and found that a number of home brewers swear by Oxiclean. They say that mixing Oxiclean with very hot water and letting the fermenter soak for an hour or two would loosen beerstone and make it easier to remove. I had purchased a 10-pound package of Sodium Percarbonate (one of the active ingredients in Oxiclean) from Amazon a couple of years ago. I use a tablespoon of it in a large quantity of hot water to remove labels from bottles for home brewing, and to remove caked on yeast from the bottom of the bottle before tossing them in the dishwasher.
I figured I had little to lose, so I took a small quantity of the chemical (approximately 2 ounces) and put it in the fermenter with the beerstone problem. I then ran the hottest tap water I could into the fermenter until it was full (or at least had a water line above the beerstone). I put the lid on the fermenter and left it for about an hour.
When I returned, I removed some of the mixture from the fermenter until I got down to the level of the beerstone. Sadly, I could still see it on the wall of the fermenter. I took out a scrubbing sponge I use to clean my fermenters and scrubbed with moderate pressure. The beerstone removed instantly. By the time I'd scrubbed the entire inner surface of the fermenter, you could no longer see or feel any beerstone in it.
If you find beerstone caked in your stainless fermenters, you may want to give this a try.