Hi,
I have a Nauticam flat port. After a few years' use it had really started to show some bad signs of water-marking. Anyway, having looked back at some old posts on the other place, I came across a recommendation for this stuff:
https://www.amazon.com.be/dp/B00ADMA8GQ/ref=pe_43847721_694249361_TE_SCE_dp_1
(windscreen polish).
It worked amazingly! A couples of minutes with the pad attached to a drill, then a few drops of water, a bit more rubbing and it was like new 🙂
The instructions say it's non-abrasive, but i don't think I'd use it on a port with a special coating or a filter.
Cheers,
Matt