OK, since I am on the road at present I pulled this off CLP website:
Cleaning gratings:
isopropylalcohol with fine tissues. You may use also methanol, athanol, standart classcleaners with attention to additional ingredients (color etc.) You shut not use aceton, gasoline, trichloraethylen. Avoid high tension or scratches during cleaning procedure.
The isopropyl alcohol should work! I just remember going into the bathroom at ILDA with Peter one time to clean a bunch of gratings with water, it always does the trick for me.