While technically possible to apply varnish to pressure-treated wood, it is generally not recommended for long-term durability and low maintenance.
Based on one reference regarding varnishing pressure-treated wood, the advice given is "I would definitely not varnish that set; you will end up sanding and revarnishing the wood every year or so." This highlights the main drawback: varnish tends to peel, crack, or fail quickly on pressure-treated lumber, requiring frequent and significant effort to maintain. The constant expansion and contraction, along with the moisture content typical of pressure-treated wood, make it a poor substrate for the rigid film that varnish creates.
Why Varnishing Pressure Treated Wood Isn't Ideal
- Poor Adhesion: The chemicals and moisture in pressure-treated wood can interfere with the varnish's ability to bond properly.
- Frequent Maintenance: As noted in the reference, you may face sanding and revarnishing annually to keep the finish looking good. This is a time-consuming process.
Alternatives to Varnishing
The reference suggests considering other finishes. For instance, if the wood already has a brown hue, it might indicate it's been stained. In such cases, the reference suggests you might not need to apply any finish at all. However, if you desire a finish for aesthetic or protective reasons, staining is often a better choice.
- Stains: Unlike varnish, stains penetrate the wood rather than forming a surface film. This allows them to move more readily with the wood's natural expansion and contraction.
- The reference specifically mentions that "for your own peace of mind you can stain it with a semitransparent stain."
Choosing a stain, particularly a semi-transparent or solid color product designed for exterior use on treated lumber, provides color and UV protection while being more forgiving and easier to maintain than varnish on this type of wood.
