I don't know what it is, but there is a brown coating on every dipper tray I have seen. It is nearly impossible to get off except for blasting. There is a concern that the sand or even glass beads used in blasting getting under the 3 baffles where they are spot welded to the main part of the tray and later coming out when the engine is running. So, I don't think you need to try to get the dipper tray down to bare metal, just the oil off and make sure there are no rust holes (sometimes they are pretty small and hard to see).