There are two methods for removing the build-up that seems to always happen on our trays--unless of course, you use the following methods after every use. If you need to remove the grease for the first time, use either of the methods below:
1. Tea tree oil is amazing..leave on for about 1-2 hours then scrub off with a good scrub brush...it WILL take a bit of elbow grease. *In my opinion, this is the most effective method*
2. Use a paste made from hydrogen peroxide and baking soda (premix in bowl then spread over area). Leave on for 2 hours then use scrub brush and rinse.
this from http://www.amumntheoven.blogspot.com/