[SV650rules]

Any acidic cleaner like Harpic will help (even coke or tomato paste LOL), you can get something really strong called 'pickling paste' (based on nitric acid I think) which welders use to clean off discolouration caused by welding SS, The rust resistance of stainless steel depends on a 'passivation layer of chrome oxide' that forms on the surface, which is often a dull grey colour. The pipe will stay shiny for a while and then discolour again, especially where it gets proper hot.

You could try green scouring pads (mildly abrasive nylon, as opposed to brown abrasive nylon which can be very abrasive) with Harpic or Jif