Hi Zachary:
My first guess would be dried silicate from a PBW type product when it is allowed to dry on the equipment. The silicate in the Metasilicate in these products can cause permanent staining. However, there are ways to remove these deposits.
Please send me an e-mail and I can give you some suggestions on how to remove the deposits. I have seen quite a few of these over the years and so far, we have always been able to get them out.
Cheers,
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Dirk Loeffler
Executive Vice President
Technical Operations
SEKA Chemicals
(404) 239-2700
loeffler.dirk@sekachemicals.com------------------------------