The performance of felts and wires is affected by their cleanliness, the permeability and airflow decrease as the felt becomes dirtier. This can be caused by ash, stickies, wet strength, biofilm, etc.

Felts can be cleaned by shock washing on a break, a machine shut or on a continuous basis using caustic, acid, low alk or chlorine dioxide based cleaners. These are applied through a spray bar and the excess sucked of through the vacuum boxes.