Here's a tip everyone can use from time to time.

If you have some sort of deposit-like stain or buildup you've tried to remove, and nothing seems to work, that is to say, you've tried water and you've also tried some sort of solvent, each with no results, then try them in combination. Some varnish type deposits will never come off if you use each cleaner separately.

Also you can use a technique employed by cooks the world over for centuries called deglazing. Heat the metal object without doing damage to it and then immediately sprinkle with a small amount of cold water and scrub lightly with a brush. While the object is sputtering and making steam is when your're at your best cleaning potential. This may work on your muffler. It might not be the best first resort, but if all else fails......

Luck to all!!!