MAN I TRYED IT ALL, I THINK IT DEPENDS ON THE SILICONE. SOMTIMES GAS WORKS, SOMTIMES CARB CLEANER, SOMETIMES WD40 AND SOMTIMES NOTHING. I GET AS CLEAN AS POSSIBLE AND TAKE MY NEW SILICONE AND RUN A BEAD ON ONE OF MY CUT OFFS TO SEE IF IT WILL STICK. IF IT DOES I SEALER UP. IF IT DOES NOT I TRY A DIFFERENT SILICONE. THATS MY 2 CENTS.