Clean your clutches, not the belt.
Ultimax belts do not require washing. That was a valid practice years ago. Our improved manufacturing processes eliminate the sticky release agent that used to cover the belt. Some manufacturers still use older technology that may require washing.
It is important to clean your sheaves. If using acetone or brake cleaning fluid on the clutch faces, be careful to let everything dry so it doesn’t get on the belt. These chemicals will break down the compounds in the belt and result in shortened belt life. Never use a belt dressing or solvent on a belt.