I would go with a solvent, soap, or other chemical strong enough to dissolve it. Gasoline may be enough, a brass brush may help (not steel). Super Clean?
I wouldn't sand the friction material at all. If the plates are sticking, it's probably going to be the tabs that stick out into the sides of the basket, or the ones that engage the inside of the basket.
Would also pull the long rod and clean it. Could maybe sand it with 400 wet, but sand it lengthwise instead of twisting the paper around it. Scratches or grooves around the rod might make it stick worse. Lengthwise should be fine. 220 may be a bit too rough. If it cleans up with soap/solvent, no sanding should be needed.
May as well clean the basket too since you'll have to take it out.
Remember, one tab of one clutch plate is offset from the others on reassembly. Could take a pic before pulling the basket.
Last edited by Spockster; 05-21-2015 at 10:15 PM.