My experience with fiber board is.....Water/moisture + Fiberboard BAD
Had a storage area that needed a cover..Just needed something quick and
it was the right size. We primed, painted, and sealed it. It didn't
last long at all. This was under an overhang, which did give it some
protection, it did get some direct weather depending on the wind, but
over all it was out of direct sun and rain.
Didn't last a year.