How to clean 24 well or spot plates sparkling clean?

I am lab. tech in a school. We use micro scale method for lots of things in Chemistry. My well plates have become little bit blackish after I used Manganese di Oxide ( MnO2) It has gone even in inter well spaces and I am unable to clean it sparkling clean. I have applied everything from Sulphuric acid to Sodium Hydroxide. But I am unable to clean them like other gl or plastic ware. Can somebody tell me how do I clean my well plates sparkling clean?