Talc will work on mild cases, but some of the soft-touch 1990s rubberised finishes are a time bomb. It may be finger sweat turns them to jelly - in which case why didn't the manufacturers test the stuff for normal human interaction - or it might rot away of its own accord. I have Sigma lenses that have turned to pure goo, you can scrape the rubber down to the barrel with a finger nail. They were kept in a Billingham with other Canon FDs and Nikkors which are spotless, while the Sigma stuff literally melted into the bag. I wouldn't purchase another Sigma on principle.