A good wood fence sealer should include ultraviolet inhibitors, include mildewcide and last between two and three years. Most wood fence sealers are incorporated into a wood stain solution that is designed to refurbish the wood. The sealer and stain combinations come in solid, semitransparent and clear stain options.
Solid sealer and stain finish options generally last the longest of the three varieties and do not need to be reapplied every year. However the downside of these types of sealers is that they are so dense that they hide the wood grain underneath and can develop a film that causes the sealer to crack or flake off over time.
Semitransparent types are the most popular type of sealer and stain finish options as they provide significant protection from weathering in addition to not hiding the natural aesthetics of the wood grain that they are applied on. The disadvantage to using semitransparent sealers is that they are not as durable as solid sealers and need to be reapplied every two years.
Clear wood treatments are generally the least durable of the wood sealant options and need to be reapplied every year. Some clear wood treatments also do not include ultraviolet inhibitors or mildewcide, which can lead to the wood fencing turning weathered and gray even with an applied clear wood sealant.