Try cleaning the area on both sides of the joint with a good detergent, dish soap, etc. first, and then apply some Vinylex to the rubber.
There shouldn't be much movement to start with, my roof clamps down tight enough so that you can actually see movement in the rear view mirror.
You don't want anything that leaves a residue as that will collect dust and dirt, which will just make the situation worse.
I believe clean and dry is the way to go.
By the way, welcome to our site, please add your location, year and model SLK to your avatar, it often helps when answering a question.....
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