I have a synth action keyboard which I'm planning to sell. As I'm cleaning my keyboard, I notice some black spots on it (possibly water spots??? I don't know how). Does anyone know what this exactly is and how I can remove them?
Yamaha keyboards of that type [PK/PSR etc] are usually painted plastic, certainly any metallic silver/grey sections - even the soft push-buttons.
I don't know of any that are intentionally patterned like that - but it's been 20 years since I worked there.
That looks to me like it's been sprayed or splashed with something that has actually damaged the paint surface, but not enough to go right through it [it's almost certainly black plastic underneath]. I very much doubt you could clean it off, it's already 'off'.
The entire top shell will be one single moulding, oversprayed in one single colour. Highlights would then be screen-printed over, or masked like the plexi-glass section you can see a hint of at the left, which is likely painted on the rear to give a high gloss face.
If the entire painted surface surface shows the same markings, it's an intentional pattern. If it's randomly varied over the surface, it's damaged.
I simply cannot tell from the pictures on https://uk.yamaha.com/en/products/musical_instruments/keyboards/portable_keyboards/psr-e463/index.html
This is what it looks like when it's worn off completely…