I suggest that you use a lead-based paint. Oh, not available! Well lead is a very good mildewcide.
Seriously, I wonder if the surface is mold infected. You stated that the paint was sanded to the wood. Perhaps sanding all of the mold and rot in the wood is necessary. Given the long term mold growth, the mold is imbedded deep in the wood. Covering active mold growth with paint is not effective in the long term. The M1 additive is effective if added correctly in the proper proportions.