Well I guess that the older I get, the slower thinking wise I get. I didn't realize that Stuart boilers, at least the 501 I'm working on, is made of brass plated copper. I should have known this but as I said I'm slowing down apparently. The odd colored patches I saw were nothing more than copper showing through the places where the brass plating had worn through. The plating must have been thin as the Maas polish took even more off the second time round. And, for what it's worth, acetone took the Rustoleum High Heat Coating right off slick as a whistle. That all said this is the end of the experiment.