If it's real nasty, use a semi-flexible putty knife to get the biggest chunks of grease off, use a cordless drill with wirecup brush, hand wire brush (the toothbrush size) and once you have gotten about all you get off that way, soak it in oven cleaner or lacquer thinner applied by paint brush a couple times and then pressure wash if you have one. That should get enough off so that you can then start to use the foaming degreasers.
