from the arctic silver website :-
2. ONLY Arctic Silver thermal compound should be between the processor core and the heatsink. Remove any thermal pads or other interface material from the heatsink before applying the Arctic Silver. Thermal pads can be scraped off with a plastic tool that will not scratch the bottom then the remnants can be removed with a xylene based cleaner, (Goof Off and some carburetor cleaners) acetone, mineral spirits, or high-purity isopropyl alcohol.
Never use any oil or petroleum based cleaners (WD-40, citrus based cleaners and many automotive degreasers) on the base of a heatsink. The oil, which is engineered to not evaporate, will fill in the microscopic valleys in the metal and significantly reduce the effectiveness of any subsequently applied thermal compound.
If your heatsink has a thermal 'pad' mounted on it, this pad must be removed before using Arctic Silver thermal compound. Thermal pads are made with paraffin wax that melts once it gets hot. When it melts, it will fill in the microscopic valleys in the heatsink with wax. To minimize the permanent contamination of the mounting surface with wax, the thermal pad should be removed before it is used and melted. Never use heat or hot water to remove the pad, the heat will melt the wax into the heatsink.
they're so anal, i use nail polish remover which is citrus based, find it works just fine.