This slows down the evaporation and thus minimizes shrinkage. If you plan of using a stain or paint, it is wise to give the surface at least six months after the initial sealing for proper adhesion. TREX is not rigid enough to be used as a structural support it is strictly for decking purposes. TREX does not require any preservative or treatment and weathers to a gray tone. TREX is available at most lumberyards and some home stores.
Plastic lumber is totally impervious to moisture so no sealing is necessary.
It is available in a wide range of colors, which is good because it does not hold paint or stain well. Finally, since there is no toxic component as with PT wood, it can also be used indoors in showers, damp basements, etc. One downside is that this product is less widely available than TREX or the other wood products. Another critical difference between these products is in the installation. Though all products can be installed using screws or nails, there are differences in each product's reaction to temperature changes.