I clean fryers in restaurants.We use a food grade spray cleaner to remove all stains. Sometimes we lightly sand with an abrasive to remove anything stuborn. One fryer ,the same one every week has a dark burn stain at the oil level line only, that is now impossible to remove. This partcular franchise has the same frying menu as others I do, yet I have this problem with this 1 fryer only,out of 100s I clean.

I think there is something different, an impurity in the metal. The fryer says stainles, but it is a very dull finish
Your thoughts please

Kitchen Appliances - Others - Pitco Frialator - SG14 Deep Fryer