John,
Here is what we have found in our research regarding "mildew". Pressure washing alone will not remove the mildew spores. Most chemicals including bleach, will not remove mildew spores. The above can remove the descoloration that the spores cause. Even if they did, you would be fighting a loseing battle by using them in your fight against mildew. Mildew spores ( seeds ) are air born, they are all around us. The more trees and bushes you have, the more spores are present. These spores will settle on a house as fast as you can clean it. Once there on a house, the climatic contitions get just right, they start to grow and will cause the same descoloration as you had before. If you apply a finish over these spores and they start to grow, you'll wind up with the grouth between the finish and the wood surface. This will continue to grow untill it breaks through the surface of the finish.
The way we have solved this problem here in the Pacific Northwest is that once we have the home back to bare wood, we apply a product called Timber-cote. Timber-Cote is a mildewcide, one of the best on the market. It will not only kill the spores that are in the wood already, but it has a residual life to kill any new spores that may settle on it. Once we apply this, we let it dry and then apply our finish. Between the Timber-Cote and the mildewcide package in our finish, we give our customer the best protection possable against the mildew returning. We offer this treatment to all of our customers as an add on. 95% of them take it. We also use Timber-Cote prior to applying a recoat.
Keep in mind that once this treatment has been done and the finish has been applied, the home must be cleaned every so often to keep the surface mildew from starting. Hope this helps.
Jim Bilyeu
Exterior Woodcare