Do you wash your own car?
Everybody should IMHO, an automated carwash wears out the soft top much faster. As part of the normal hand washing process you start at top of the car and work down to the wheels and last thing you do with the drying towel is give exhaust tips a good wipe. Do that and they stay bright. Otherwise, even when tips are stainless that does not mean 'stainfree' and contaminants will build up and leave rust specs. At the end of the process of washing and drying the car the towel will be slightly dingy and perfect for giving those tips a quick wipe. Do it often and they stay looking decent. Never do it and they will be tarnished and dull and speckled with rust specks quick even when the covers are stainless (which is why they call it 'stainless steel' and not 'stainfree steel'). The metal is not sealed, it will pit if you neglect it long enough. If you have a lot of buildup, grab some metal polish and apply some elbow grease and they will shine up quick.