Staining on ES Sink

Just got a call from a customer that we installed with a Pegasus (Home Depot) black ES/quartz sink.

He claims that the sink has a "lighter" appearance around the drain flanges when it dries (looks OK when wet). Have any of you ever heard of this phenomenon? Could it be the plumbers putty?

Any suggestions for a possible cleaner or other solution?


Thanks in advance for the help.
_________________
Mark Meriaux, SFA/MIA
The Granite Shop
www.theGranite (http://tanbrown.org) shop.net