To prevent mold spots from developing under the stain, these mold spores need to be removed, preferably by washing the surface with a solution of Log Wash (2 cups per gallon of clean water). Even if glass, dry ice, or other types of blasting media is used, we still recommend washing down the blasted surface to remove any remaining foreign matter substances that may be adhering to, or embedded within, the wood. Blowing high pressure air on the wall will not totally remove all of the dust and old finish residue; and without actually washing the surface, you run the risk of compromising the adhesion of the new finish.
On the other hand, most chemical strippers and cleaners do a good job eliminating mold spores and dust particles which is why it may not be necessary to use Log Wash if the home is going to be stained within a few days stripping or cleaning. However, if the stain application is delayed more than seven days after the wall has been chemically cleaned, an application of a two cups per gallon Log Wash solution should be made, rinsed, and the wood allowed to dry before the stain is applied. It only takes a few days for enough mold spores, pollen, and dirt to accumulate on bare wood to create a problem.
We always recommend thorough cleaning and preparation procedure because careful preparation is the best way to help prevent unsightly fungal growth under our transparent finishes, and to ensure the best adhesion and longevity of our exterior stain and topcoat systems.