I am renovating a small sailboat (catboat type). I have to build a
centreboard for it so the plan is to shape a 3' X 1' X 3/4'' board and
put a protective coating on it. Would marine quality paint good enough
to provide a protective coating or should I really paint the thing and
then apply resin on it?

If resin is the only decent solution, is there a type that can simply
be applied with a paint brush rather than having to deal with the
fiberglass cloth and go through all the complicated process of applying
it?

Just for a contextual refernce this is just a tiny 11' sailboat which
will be used primarily just to play around at our cabin when the wind
picks up. Probably won't be in the water more than 10 times per
summer.

Andre