Fill A Bucket With Chlorox
JW,I have the same vents on my boat. They were getting all nasty looking and someone suggested filling up a bucket with chlorox and letting the vents soak. Worked great! Really. Definitely worth the $4 try before you sink money into anything new. Takes about 3/4 of a gallon. Soak them for an hour each, one at a time, completely submerged. Take them out, rub them down a little, and they difference is quite substantial. No more black stainy stuff. Just white.Good luck.Joe Mullee