MDO, without paint will last as long as if painted. ask any concrete company that makes forms with it. they oil the forms/boards as they install em. my question is: WHY would you not paint it, if you are making a
SIGN? it really is an ugly brown color if left unpainted. as for the back, again, why would you NOT WANT TO PAINT it?
unless its going up against a wall and you wont see the back.