To do it properly, open the cavity where the pots are and spray contact cleaner (Servisol or another brand, not WD-40 into the opening of the pot. This is the base where the wires are atached. Then rotate the pot a few times and see if the scatchiness reduces. If it does then repeat and the problem should go away. If it doesn't then you will need to replace the pot/s.
You can try Luther, or Sim Lim if he has none left.
cheers,
Mark.