Outdoor wood furniture is subjected to expansion and contraction due to frequent temperature changes and may be damaged by the sun's ultraviolet rays. Oil finishes penetrate into wood fibres, enhancing the beauty of the wood but offering little protection from the elements or from heavy use. Varnish forms a protective surface layer that provides durability to furniture exposed to the elements and subjected to the rigours of outdoor activities.
Oil finishes include linseed oil, tung oil and oil-varnish mixes that may be marketed as Danish Oil or Teak Oil. Boiled linseed oil includes dryers to reduce the amount of time needed between coats. Oil finishes are applied by wiping the oil onto the wood in the direction of the grain, briefly waiting for penetration and then wiping the excess with a lint-free cloth. Subsequent application requires complete drying, making the process rather time consuming.
Varnish resists wear, moisture and heat and is therefore the preferred clear coating for outdoor furniture. Most varnishes will cast a slight amber hue when they are applied and will yellow over time. You may wish to avoid using varnish over light-coloured wood to avoid its yellowing tendencies. The best varnish for outdoor use is a high quality spar varnish. This product is far more resistant to wood expansion and contraction and many also contain ultraviolet inhibitors to mitigate sun damage.
All varnishes are not created equally. Most are made with vegetable oil and resin. Modern formulations, considered to be synthetic varnishes, include alkyd, phenolic and polyurethane. Varnish is brushed on rather than wiped on and may attract dust during its rather prolonged drying time. A dust-free working environment is therefore essential. Each layer of varnish is distinct. Careful sanding is required to work out surface blemishes and to provide a good surface adherence for each additional coat.
Spar varnish is a type of phenolic varnish formulated for use on wood that will be exposed to the elements. You may have seen vintage watercraft made of mahogany and restored to an attractive, durable shine. The boat was most likely treated to multiple coats of spar varnish. Clear wood garage doors also last far longer when treated and maintained with spar varnish. Your outdoor wooden furniture will remain gorgeous for years if protected with high quality spar varnish.