There is a beer stone remover that Five Star sells. I called Five Star awhile ago and asked them about beer stone since there is so much contradictory info out there, and not much info and products for home brewers. They advised that the best thing you can do after a hot PBW soak is to then rinse with hot water, around the same temp as the hot PBW. Then after rinsing with hot water you then spray on an acid like Star San and coat the inside and then let it air dry with the Star San coating. This will keep the beer stone at bay. I also learned that if you soak the kegs in PBW that you don't do it for more than 24 hours. If you do, you will get white deposits on the metal. I recirculate hot PBW in my kegs with my home made keg washer (with a pump) and then rinse hot, and then spray some Star San in the kegs and let dry. Works great. This will keep them in great shape and then before kegging my beer I will rinse and recirculate Sani Clean for 15 minutes and then shake it all out and then keg.
For my conical fermentor I soak it in hot PBW and then hot rinse with water and then spray Star San and let dry. This what I do everytime now. No more beer stone build up. If you have some build up you can use the beer stone remover they sell and that should clear it up for you after a PBW hot soak.
Hope this helps......
John