Oh man. Thats why I hate that rubberised crap they insist on using. Same thing happened to the mod wheel on my moog. Total gunk fest. I took the wheel off, soaked it in methylated spirits and scrubbed it all off. Yuk.
Theres just no need for that rubberised coating.
Id take the button caps off, clean them. Dry them properly then refit. Pain in the neck, but an easy enough job.