You should never powerwash your logs at close range. This drives the water so deep into the wood fiber it will never dry out properly. Spray a soapy solution of warm water and Dawn dish soap on the walls and scrub it with a SOFT brush then rinse with the powerwasher staying at least 6′ feet back. To apply borate you need to have bare wood to make it absorb into the wood. Corn cob blasting is the best way to remove old finishes since it is a dry process. If a borate is applied correctly it will last for years.