Before applying or re-applying anything, such as a finish , preservative, or even a sealant like a chinking or caulking, you should clean the logs thoroughly. A good cleaner will effectively remove dirt and pollen, some molds, mildew, and loose coatings; but will not remove old coatings that are still well attached to the wood. For that you need a stripper, media blasting or sanding.
Chlorine Bleach (household bleach) is vey effective in bleaching wood and killing milew; and is also inexpensive. But chlorine bleach will kill plants, and can over-oxidize thereby degrading wood cell walls. It also can corrode nails and screws. If not rinsed completely from the surface, chlorine bleach can degrade any coating that you apply.
Herein, we sell two sodium percarbonate bleaches (Oxygen bleach) (CPR and Defy Wood Cleaner) and an oxalic acid (Defy Wood Brightener) . Oxygen bleach is environmentally friendly, breaking down into hydrogen peroxide, soda ash and water- non-corrosive little effect on subsequent coatings. It cannot be use on redwood (turning it black), which is why we also have an oxalic acid (Defy Wood Brightener) available. VIDEO VIDEO VIDEO VIDEO