Pressure treated decks are the least expensive option, however, the wood will crack, split, and possibly even twist unless you take precautions, by Daggett Builders, Inc
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PRESERVING TREATED WOOD
Source: http://www.schultzinspect.com/html/treated_lumber.html
Even though lumber may be kiln-dried before treatment to approximately 19% moisture content, using water-borne preservatives increases the moisture content – possible to as much as 75%. This causes the wood to initially swell then shrink as it is exposed to the air and the wood comes into equilibrium with the environment. If this shrinkage is too rapid, as is often the case with full sun exposure, the wood is likely to crack and distort.
A water-repellent sealer should be applied at the time of construction, or as otherwise recommended by the wood or sealer manufacturer, to help slow this initial shrinkage and prevent further damage from wetting-drying cycles. Some treated-wood products are now available with water repellency built in through the addition of water repellents to the preservative solution.
Too much moisture in the wood, however, may prevent the stain or paint from penetrating the wood sufficiently. It is best to test the wood by painting or staining a scrap piece to see if it applies properly. If not, wait until it reaches moisture equilibrium. While some latex paint products are available for finishing treated wood , semi-transparent, oil-based stains work best. A water repellent should be applied annually. If you do not have time to wait for the wood to equal equilibrium you can purchase wood marked “KDAT” (Kiln-dried after treatment). This wood should be ready for an immediate sealer treatment.
But all treated wood should be cleaned and re-sealed yearly to maintain optimum appearance.
Unprotected lumber will also begin to change color and darken as a result of the wood’s reaction to ultraviolet (UV) rays of the sun. Sealants with UV inhibitors will protect the finished paint film from deterioration; those with a UV “absorber” are more effective in minimizing changes to the natural wood pigment itself, an important consideration when using clear finishes.