The guys at the mens shed use flap discs on wood & when they lean too hard they get clogged with burnt wood so they chuck them in the bin rather than clean them using the rubber cleaning bar we have. 
I rescue them when I can, take them home & if you run them on steel it usually removes the burned wood & restores them to usable condition again.

I rescue them when I can, take them home & if you run them on steel it usually removes the burned wood & restores them to usable condition again.