Cover any nearby plants or patio furniture with a plastic drop cloth.
Apply a wood cleaner to the wood according to the package directions, which will vary with the brand. A wood cleaner will remove stains and mildew growth from the pine.
Set the pressure washer to 1,500 PSI or less. Install a wide-angle nozzle. Keep the power washer 18 inches from the pine, and spray the fence from top to bottom to rinse off the cleaner and remove any loose fibers.
Let the pine fence dry completely. This will take three to five days, depending on your climate.
Open the can of exterior wood sealer and stir with a paint stirrer. Apply the stain to the wood, one plank at a time, from top to bottom. Use long, even, overlapping strokes with a natural bristle brush if the stain is oil-based. Use a polyester brush for acrylic stains.
Let the stain dry according to the package directions. Apply another coat for a darker finish, and let that coat dry.