I never tried this on my Rock (stock pipes) but on a VT1100 with straight chrome pipes I had the side of my boot against the pipe for about an hour before I realized it. It left an ugly, crusted bit of black about the size of a silver dollar on it.
Got it home, showed my wife and kind of off-handedly she suggested that I use oven cleaner. I'll be dammed, it worked!!!
Run the engine to heat up the pipes.
Put on a heat resistant glove.
Spray the oven cleaner, let it sit and wipe.
I had to wipe hard for a bit but it worked. After I let the pipes cool, I used some mild chrome polish and viola! All better!
Karl