Billy, New Orleans Atmosphere is of course one that is difficult to treat. Everything down there has some sort of growth on it. I remember that famous cemetary was almost green with growth on the stones.
You know, you have tried all that I can suggest, that flat paint thing is weird.
Billy, you know what you might want to do. Seek the councel of a Sherwin William's Tech and entertain your situation and location and see what they have to say. There has to be something out there for that locale that will work. I cannot supply you with an answer other that what is above. We do not have that climate and we are dry all the time.
I am sorry for giving you a dry answer, but that is all I have.