You can buy silicone remover from places like screfix and B&Q. Not sure how well it works on Sika because never had to use it
The point about a very sharp wood chisel as opposed to a craft knife is that if used ground face down you get very good control over how far above the surface you are cutting (a bit like woodwork really ). It takes a few attempts to get close to the roof surface but it's easy & doesn't do damage.As previous I use a very sharp old wood chisel ...delicately .residues can be cleaned with cloth white spirit and small amount of acetone ,re-sica with new unit clean new residue excess with white spirit jobs a good-un and of course a bit of elbow grease
It's not painted on this van but GRP anti hail skinFirstly it will probably make more of a mess than it already is , if you remove it mechanically it will more than likely chip the paint off the roof panel.
Secondly , I suggest you leave it alone , after all no one except you will ever see it on the roof and if you sell it you would be very unlucky to find a buyer that makes an issue of it.