Honda FOURTRAX 2018 Owner's Manual page 201

Appearance Care
Exhaust Pipe and Muffler Maintenance
Refer to Safety Precautions on page 103.
The exhaust pipe and muffler are stainless steel but may become
stained by mud or dust.
To remove mud or dust, use a wet sponge and a liquid kitchen
abrasive, then rinse well with clean water. Dry with chamois or a soft
towel.
If necessary, remove heat stains by using a commercially available
fine texture compound. Then rinse by the same manner as removing
mud or dust.
Servicing Your Honda 191