When I go my Fender Rumble 40 home after buying it secondhand, the first thing I noticed was that the knobs were unpleasantly sticky which did not creat a great first impression of an otherwise great amp. I decided that my microcube better fits my needs for a practice amp, so the rumble is now on reverb. I really don't want the new owner to feel ookie every time they adjust the settings so the stickyness had to go.
I appears that the cream knobs on the amp were given a rubberised coating which has broken down causing the stickyness. I thought about trying acetone (nail polish remover) to get this off, but was concerned that this might attact the actual plastic of the knob. I figured that surgical spirit might work so bought a small bottle (£2.19 from superdrug). Wearing latex gloves, I soaked the knobs in a ramekin filled with surgical spirit for 10 minutes, then scrubbed each one with a spare toothbrush and rubbed the surface with kitchen towel. The result is that all of the sticky rubbery gunk has now gone, the knobs are no longer sticky, and importantly this hasn't changed the appearance of the knobs.