Discolor or even burn through the piece. A friend of mine got a kit from Eastwood and tried it. He ended up burning right through the piece after it started to blue and then he said it acted like a welder and burned right through the spot.
Either your friend is a complete idiot who should not be allowed near power tools or this is total bullsh!t.
I think its the latter ... there's no way a buffer will burn through a piece of stainless ... the buffer would catch fire long before that happens.
