YES, you do need to paint both the Centerboard (IT IS NOT a "swing-Keel", that is a totally different type of setup, what you have is a KEEL/CENTERBOARD, aka: a shoal keel that houses a lightly ballasted or unballasted CB.) and the inside of the centerboard trunk in the keel. However, I would think that if you were able to paint the whole CB and at least as much of hte slot as you can easily reach with the CB at least partway down, it will be enough. Be sure to clean out as much of the fouling and junk from hte inside of hte CB trunk first, powerwashing the CB slot may help, although it may drive the gunk deeper. I remove the CB from my DS II every few years to give hte Cb and inside of hte CB trunk a real good cleaning and at least one good coat of paint (2 coats is better, but don't build up too many coats over time). Removing hte CB from the 25 is not terribly hard from what I've heard (not quite as easy as on my boat....but not really all that much harder).
I keep my CB up most of the way while moored and that seems to reduce fouling as well as preventing the CB from banging back and forth as the boat rolls. I use a thin piece of metal strapping to scrape between the CB and the sides of the CB trunk each Fall to try to clean out as much gunk as possible before it dries (OK, I haul in October, and the CB is sometimes still damp in April when I go to paint it). That helps to keep the CB free of jamming.
