Having just finished with the coating on my concrete board, I'll pipe in with my 2 cents. I'd at least put a coat of surface bonding cement on it. It has good weather resistance and will seal up any of those small holes where perlite will evenutlaly find it's way to pour out....
I used surface bonding cement to coat mine. I put it on approx 1/8 to 1/4 inch thick and used the screen tape on the corners and edges. It mixed up nice and creamy and was easy to apply - two things to note here - wet the board a few times before applying - that stuff really sucks up the moisture and second - apply very thin coat first to make sure you have good contact between the concrete board and suface bonding cement.
Good luck!
Christo