So I took the belt off to scrap it but decided to try a way of cleaning it. The way I did it was so simple [in this case with this timber anyway] I have got the clogging with rosewood in the past but never tried this.
I folded the belt over a sharp radius, in this case the arm of my table saw fence rail. I then pushed and pulled the belt in a stropping action and the resin build up fell off. In the other photo I have folded the belt back on its self and got the same result. Some stubborn bits needed a fingernail action on it. I then replaced the belt on to sander and run it against a belt cleaning stick, new life into an old belt.




