The amount of preparation on the wood depends on the condition of the wood. Remember when you are finished applying Hemp Shield, the wood will look the same as it does after the prep. I find a good place to start is with a power washer at about 2500psi and an 8-12 inch fan tip. Start at the top log and power wash down the length of the log. A mild soap solution can be added by way of the pickup tube most washers come with. After the logs are washed clean and dry completely, you can address the mold, mildew issues. Many product work for this well and they there are some form of wood bleach or oxygenated cleaner (Clorox or Oxy-Clean chemistries). You must “sterilize” or kill as much of the mold and mildew as possible before coating the surface. Failure to do this will result in the mold and mildew growing and spreading under the finish. This is a scenario you do not want to deal with. Use the cleaner in sunlight. The UV rays (do not have to hit the home directly) will break the double bonds of the bleach and release the material into the wood and be very effective. On a cloudy day, this effect is muted and less effective. Now you have a clean and sterile wood to finish. Decide on a color or clear and coat a test patch, Let dry overnight and see if this is the finish you are looking for. Remember all wood darkens some over time.