I've had quite a few SR mags where the rubberized plates turn white and often tacky. Never found a good solution for cleaning,  but I had a 'junk' plate and tried some Rubber Renue that I typically use on capstan rollers on tape decks. Seemed to work great - got rid of the white, cleaned dirt and left low tack. You really need to use LINT FREE wipes for this - not paper towels. I used Kimwipes. MG Chemicals 408C Rubber Renue & Kimwipes Left is untreated mag, right is treated - it l