Rubber Coating on Buttons

[Fin25]

Digitakt keys sticking dirt

Hello folks, I need your help! Something happened to the keys of my Digitakt and now the rubbery surface of my keys are stick dirt on them. When I tried to use a cloth its fabric got a bit glue to the keys. I don’t know what exactly caused that (maybe sweat?) but it happened after 3 years of (not so heavy) use. I’m trying to figure out what kind of thing I should use to clean it up. Any suggestion will be appreciated. Thank you, [20210517_212632] [20210517_212622]