Do nothing or sand face of plastic plate on layer Aluminum with fine sandpaper (just one small part are or all surface (better) but not too much (will scratched surface plastic disc -
-Put it in pot muriatic acid in 5 minute or more and see all layer of metal(ALU) has been removed.( ̣outdoor and safety eye and hand please) -
-washing with water.
My friend tell me :
*Lye drain cleaner that can do such things but I have not tried
* If you succeed with other product please let me and folk to know please