Last year we had our porch remodeled. The ceiling is Pine Bead board. When new last year I stained the ceiling with Minwax Polyshade, which turned out great. However, the ceiling gets mildew on it from time to time (southern humidity). At this point which would be the better option, to coat with Helsman Spar Urethane as an outdoor protector to combat the mildew, or apply something like a Stain Sealer (Olympic, Cabot, etc) that is meant for Decks and fences that resists mildew?

I really don't want to strip the original stain as the concrete floor has SunDek on it & I don't want to risk damaging the floor coating with a stain stripper on the ceiling.

Any advice is appreciated.