Pressure-treated landscape timbers have a preservative coating that protects the wood from common issues, including infestation by termites, weather-related damage and rotting and decay. Though this coating remains solid on timbers that are uncut, if you need to cut the pressure treated timbers, the wood in the cut areas become exposed. When this happens, the wood once again becomes susceptible to common wood issues. You can help keep the timbers protected by coating and sealing the cut areas.
Wash the cut area on the pressure-treated landscape timbers with oil solvent, which you can buy at many drug stores. Rub the oil solvent onto the area around the cut with a rag, applying enough pressure to buff off most of the oily feel.
Sand the area around the cut area of the pressure-treated landscape timber with sandpaper to remove the remainder of the solvent. It may take several sheets of sandpaper to remove the oil layer in the area, since the sandpaper is likely to gum up during the sanding process.
Stain or paint the wood if you want to while the cut area is exposed. According to the University of Minnesota, pressure-treated wood can be painted and stained like any other wood.
Apply alkaline copper quat or copper azole to the surface of the wood once the paint or stain has dried. Both of these chemicals preserve the pressure-treated landscape timbers like the original oil applied to the timbers. Use protective clothing and a respirator mask during application, and apply the chemical with a paintbrush or rag.