As title.. I was picking up my mug of tea but clumsy dropped it spilling tea everywhere and taking off a slither of the whatever the shiny stuff is that coats the piano case. The mark is in an obvious place, on the 'lid' that covers the keys. I've tried rubbing with a microfibre cloth but the mark is white so is very obvious even though it's only about 1x10mm big. I thought about black shoe polish, but that would leave an obvious dull spot. Help!