Manufacturers of pressure-treated wood use chemicals that resist rot and decay, and because these chemicals also resist finishes, the manufacturers used to advise against sealing the wood. That isn't true anymore, at least not for all types of pressure-treated wood; the label on the wood usually specifies whether sealing is recommended. Semitransparent stain seals well, gives good sunlight protection and penetrates well enough to make it suitable for both vertical and horizontal surfaces.
Choose a semitransparent stain or clear sealer. Solid stains and paint give the best UV protection, but they don't bond well with the chemicals in pressure-treated wood. They weather and wear unevenly, and your deck will soon need recoating.
Check the wood for mildew before you seal it. Mildew is common on treated wood because the chemicals used for pressure-treating retain moisture. Clean off the mildew by scrubbing with a solution of 1/2 cup of chlorine bleach per gallon of water. Rinse the wood thoroughly after the bleach treatment.
Clean all deck surfaces that will be stained, using a commercial deck cleaner and following the manufacturer's directions. Most cleaners work best if followed by a vigorous scrubbing with a scrub brush. Rinse the deck thoroughly and allow it to dry for at least two to three days.
Apply the stain or sealer with a paintbrush or sponge applicator or by spraying. An easy and effective way to spray clear sealer is to use a garden sprayer. If you want to spray semitransparent stain, use a compressed air or handheld airless sprayer. Wipe up any pools of excess stain as you work. Let the stain dry completely. Apply a second coat, as applicable.
Maintain the deck by applying a fresh stain or seal coat every year.