Cleaning Exhaust systems:
Pipes, headers, mufflers, etc., can all be cleaned with 0000 grade of steel wool. Use of 000 grade should be reserved for the worst cases. Exhaust pipes polishes to reduce bluing are available, most do little, and the bluing tends to come back soon. A particular problem is that exhaust systems tend to get oily grunge & other fun stuff such as burned boot heel material on them that gets thoroughly baked onto the surface. Late model Airheads have pre-muffler/collectorson which can can be ‘wire brushed’ with a steel or brass wire wheel/brush, but they are more abrasive to the surface. Brass bristled brushes are safer. Wire brushes may be your last resort if items are truly a big mess, this is particularly so if your items are chrome plated steel & are “rusting”. Strong chemical means, which will not eat the metal like steel brushes will, work well for heavily baked-on oil/grease/etc. I HIGHLY recommend the entire process be done OUTDOORS.
Clean off any oil/grease you can, with a petroleum solvent. Kerosene; paint thinner; Stoddard solvent; MEK; Acetone; whatever you have. Wash with very hot water & strong dish washing detergent mixture. If the items are still mounted in your bike, you want to mask off, by whatever good means, the bike from the next step. You do NOT want any aluminum castings getting strong ‘oven cleaner’ on your castings, etc.
Obtain a spray can of OVEN CLEANER. Get ONLY the type containing sodium hydroxide (common household lye). Rarely, these days, you might find potassium hydroxide, it is also OK. NO OTHER TYPE of chemicals but these two provides seriously strong chemical cleaning against burned-on grease/oil. Sodium bicarbonate is NOT going to work hardly at all.
If you can, warm the parts to be cleaned. The hotter the parts, the faster the stuff works…but, NEVER higher than water boiling temperature. YOU ABSOLUTELY MUST WEAR SAFETY GOGGLES!! If any spray gets on you, WASH IT OFF, right now, NO waiting. KEEP IT OUT OF YOUR EYES. Try hard NOT to breathe the spray. Spray the surface generously. DO NOT BREATHE-IN, WHILE SPRAYING. How fast the chemical works depends on temperature and that it remains wet. It is OK to use a foggy spray of water to keep the chemical wet, if it is showing signs of drying. In particularly egregious cases you want a thick layer plus you want it covered so it will stay wet, perhaps overnight. The chemical must be wet. It is better to do this job with the parts off the bike, but that may be inconvenient. After a while, perhaps 15 minutes or longer, wash off very thoroughly with water. Use a stiff floor scrubbing type of brush or modify a stiff paint brush by shortening the bristles so it is even stiffer. If the wet surface feels soapy/slippery, it is not washed off enough.