Depending on the size of the area to be cleaned and how easy it is to get to I would go to Lowes or Home depot and get some Acetone and Rubbing alcohol followed by a ton of cleaning rags. Start with the Acetone and break down as much grease as possible, get a propane torch and heat the area to be patched to get the remaining grease to flow and expose itself. Now use the alcohol to do the final cleaning, you might have to repeat the heating with the torch and wiping with alcohol and clean rags a few times to remove all the Goo. It will need to be spotless before applying the silicone or it won't stick and seal. You might want to consider some JB Weld instead of Silicone, just a thought. Be cautious when using the Acetone, it will create explosive fumes inside your grill, air out the inside with a fan before you fire up your torch to heat the patch area.