Pressure treated wood has wood preservatives forced deeply into the fibres to make it virtually immune to rot and termite damage. It usually has a slightly greenish cast and costs a little more than untreated lumber. New pressure-treated wood decks should be sealed to preserve colour and appearance. There are special considerations to sealing, staining or painting new pressure treated wood decks.
Let your new pressure treated wood deck weather before sealing. This depends on the type of sealer you plan on using, so ask at your local lumber yard. Some clear sealers can be used right away. Most manufacturers recommend waiting about 2 months before using an oil-based stain. Painting or water based staining should not be done until your deck has weathered about a year.
Reconsider painting your deck, if that was your plan. Paint or solid acrylic stains require ongoing maintenance and can end up being a lot of work. Should you ever decide to change from paint to natural looking stain, it will be almost impossible to remove the paint, especially if you have painted the railings as well. If you insist on painting your pressure treated deck, wait a year and then use the very best paint you can buy. Consider using a marine-grade paint for optimum durability. It will cost more but may be well worth it.
Use a clear sealer on your new deck right after it is built. Contact the manufacturer or the lumberyard for a list of approved clear sealers. Usually brushing or rolling and then brushing out is the best way to seal your deck. Use a big enough brush to get the job done quickly. Brushing works the sealer into the wood fibres better than spraying or rolling.
Use an oil-based semi-transparent stain for the best appearance and durability. Usually you have to wait about 2 months to allow the wood to settle and weather before applying the stain as a sealer. Follow the directions for sealing above and always work with the grain of the wood.
Follow the manufacturer's directions on the sealer or stain. Most only require one coat and then a recoat the following year. If the directions call for only one coat, don't think that by doing two coats you will get a longer lasting job. You will end up with an uneven appearance and possibly even peeling stain or sealer.
Recoat at least the horizontal parts of your deck every year if using a clear sealer. Most clear sealers are not effective for much more than a year. Depending on the size of your pressure treated deck, it may be more economical to buy a 5 gallon bucket of sealer and keep it in a warm, dry place until the following year.
Since some of the chemicals used to preserve pressure treated lumber are potentially harmful, sealing is a good idea because it seals these chemicals in.
Always follow manufacturer's recommendations and directions on the can of sealer. There are many different types of sealer and application and usage directions differ for most of them.