If it's really bad and the front pipe has gone Blue get an angle grinder and put a wire cup brush on it, then soak the exhaust in WD40 and skim over it with the angle grinder.

DO NOT use angle grinder on the polished boxes or the tailpipe, ONLY the main pipework.

Then wet n' dry sand paper ultra fine THEN use Solvol autosol and it will be like a mirror.

Don't waste autosol on your exhaust to begin with you'll use tubes of the stuff and it's expensive