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The plastic pan should also work using the ammonia cleaning method (put the grates in the pan, pour in some household ammonia - you don't have to cover the grate - and seal everything in a plastic bag for a few hours).
I think I just hosed off, may have used a plastic brush to remove what the ammonia had loosened. My wife uses the same process to clean the burner pans on our kitchen stove, and they come out looking like new. Here's a link: http://www.ehow.com/how_4868839_clean-bbq-grill-ammonia.html