If your fence is new, apply a preservative designed for freshly pressure-treated wood. Wait two to three months before painting to allow the wood to dry, and doing so will give you a better finish. Check for any boards that are damaged, and replace them if necessary. If your fence has been around for a while, skip to Step 2.
Prepare your fence for painting by washing off any accumulated dirt and debris, such as cobwebs. Sand down any rough spots, and remove any paint that is chipping or peeling.
Work on one section of the fence at a time. If staining, use stain designed for use on pressure-treated wood. If using paint, apply two coats of acrylic latex primer, followed by a coat of exterior all-acrylic latex house paint. Buy primer and paint that is designed for use on pressure-treated wood.
Paint one board at a time, starting on the top edge and painting down the front of the board and one side edge. Repeat this step until every section is completed, then go back to the first section and paint the back of the board and the unpainted side edge. Do this in every section until the entire fence is done. Use the 4-inch brush for most of the painting, and use the 1-inch brush for touching up spots that the 4-inch brush misses (like the areas where the boards are nailed together).