I've had this problem on my 2014 XKR. It affected the buttons on the steering wheel, doors and centre console. All started at about the same time, when the car was 5 years old. It is not about dirt or sweat. The thin rubber soft-touch compound begins to degenerate after a time. Sometimes the process is accelerated by exposure to UV but on many cars it occurs even if the car is always under cover. You'll see the same thing happen on binoculars etc. It cannot be cured by covering it up or by converting it. The only method that works, short of replacing the part, is by removing the affected layer. If you are lucky, there will be good rubber underneath, or another hard surface which you can live with. On my car, I was lucky that my method worked without removing the white lettering on the buttons, but that is a risk you can't avoid. The method which worked for me was to take a cleaning cloth (one with a bit of nap works best), then apply isopropyl alcohol to it and rub until you feel and see the bad rubber start to bead up just as if you were getting rid of old gum residue. Citrus gum remover also works. Makeup remover, mentioned by some others, probably does the same thing, but it will still require physical removal. I wouldn't try acetone, but was tempted to after 30 minutes on one section.
The removed rubber will smear across to adjoining parts so keep moving to clean cloth and pick up he bits. For fiddly bits I used cotton buds, but it's difficult to get enough force on them to work, as this process is mainly physical, not chemical. You'll feel it has worked (or not) when the surface doesn't feel sticky when you rub dry cloth across it. You have to take care with buttons (don't break them) and corners (you may break through to underlying material) so it's a slow process. To finsh off, I used a little armour-all, in the hop that this might slow any repeat degeneration, but it's not really needed.
Hope this has helped.
