I had a similar thing happen with my R1S. What worked for me was Meguiar's PlastX. It's a dedicated plastic cleaner, and I've found it effective on various types. After the cleanup, I applied 303 Aerospace Protectant. It's like a shield against mud and dirt buildup. And I’m quite satisfied with...
For the best results, I'd suggest going for a super flexible, top-notch PPF material. Before you apply it, make sure to give the shark fin housing a good clean and degrease. Any leftover gunk could mess with how well the PPF sticks and how long it lasts.
Grab a heat gun and gently warm the PPF...
I've had success with 303 Aerospace Protectant. It leaves a nice, clean finish without streaks, and it also provides UV protection for the plastic. Or, you could mix a solution of water and a bit of mild dish soap.
One trick that has worked for me is using a mixture of isopropyl alcohol with water (1:1). Apply it with a microfiber cloth, let it sit for a bit, and gently wipe it off. This can effectively dissolve water spots.
Another trick is using a clay bar made for cars to remove contaminants, including...
Go for a gentle wax that goes well with the plastic coating. I've had good results with 'Meguiar's Ultimate Liquid Wax' because it's safe for protected finishes.