Since you ask for it, I torture you some more !
I first cleaned the surface with sandpaper.
Put a chrome layer on it, but that paint didn't give the chrome shine unfortunate it was silver.
I've put hard varnish on it, but to many layers, so it gave a reaction effect, since the chrome-silver layer below it stayed completely wet and sticky.
So I layed the keytar case in the sun for hours, afterwards it was hard. And I think now it's completely dry.
It's a worn effect that I wanted, so no problem that it has so many ripples and scratches.
The rust paintjob was done by my wife.