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patience, a stiff solvent brush, mean green or purple stuff spray bottle, a putty knife, and lots of rags.
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I just went through this with my car (well, a good part of it anyway) and I have to agree with Mark on this one. Get you a "grease cutter" like Simple Green, 409 or Now. Soak the grime and let it sit for a few mintutes, then scrap it off with a cheap plastic putty knife. Finish her off with liberal doses of grease cutter, a stiff plastic brush and paper towles. If possible, catch all your scrapings on a tarp or flattened box or something then dump them into your oil catch pan for the next time you take it to the parts store to be recycled. If you have a particulary nasty spot, denatured alcohol is much safer for you and the environment than mineral spirits or gas.