I am going to pull boat from water to free up the center board. I would like to know what have other owners for similar boats have done for cleaning the center board well and applying an anti-foul coating in this narrow opening. Note: There is no access from inside the boat.
This isn't much of 'how to' forum, but I will I'll give you my 2 cents worth because I had something of the same porblem on a boat I used to own.
I had to REMOVE the centerboard. I don't know how easy that will be with your particular boat. But if you do this, make sure the boat is blocked in such a way that you have enough clearance to get it out.
Cleaned up and painted CB. Then fasioned by own scraper/ sander tool which was on the end of a stick to clean and sand the inside of the trunk. Then painted inside of trunk with a foam pad taped to a stick. You also want to use some sort of ablative anti-fouling paint so you don't ever have to sand it off again.
It wasn't that difficult, but this was a small boat.
This is the major drawback of keel/centerboard boats that need to be left in the water for any length of time.