Tried using a utility knife blade as a scraper and had the same issues as the Xacto knife. Tried the Scotch-Brite kitchen scrubber suggestion and it worked the best out of all the methods I've tried so far. The combination of mechanical scrubbing and acetone seems to work well at removing the silicone on my case and the thin black layer of corrosion(?) that appears under it. You can compare the finished and unfinished portions in the photos. This was a brand new scrubber when I started.