Power washing a fence is important before staining. Some fences have existing paints on them. Even if the paint is scrapped, the leftovers need to be washed off to allow the new stain stick. When a fence is power washed, the stain can sink in and penetrate well into the fence. If not power washed, then the stain won’t be absorbed into the wood. And over time, the new stain may start to peel off.