Spar varnish is a high-gloss product that is used for outdoor applications, including wooden furniture. It is also known as "marine varnish." This type of varnish is made to provide protection from changes in temperature, as well as the sun's ultraviolet rays.
Wood is a living product, and it responds to temperature changes by expanding during warm weather and contracting when the mercury in the thermometer dips. Varnishes made for outdoor use must have a certain degree of flexibility, or the finish will crack. A spar varnish is also made to be water-resistant, since exposure to moisture will cause wood to rot over time.
Light amber in color, this finish is made with tung oil and linseed oil, as well as phenolic resins. The better grades of spar varnish have a higher level of tung oil in the product. Some manufacturers produce a product that doesn't contain any linseed oil at all. Linseed oil does break down over time, and causes the finish to deteriorate.
Before using spar varnish, read the instructions on the container carefully. They will indicate how long the product will take to dry. Another important bit of information on the label is the recoating time. The user will need to give the varnish some time to cure in between coats, and it's better to apply several thin coats than one thick one.
Another tip that will be in the instructions is whether the product should be thinned with mineral spirits before use. Adding between 5-10 percent mineral spirits will help to make the spar varnish flow off the brush more smoothly. Pour some of the varnish into a separate container before adding the mineral spirits. Do not return the thinned varnish to the original container after use.
A good quality bristle brush should be used to apply this product. Look for tapered bristles when choosing one. Dip the brush into the container and press it against the side to remove any excess product before brushing it on. This product works best when applied to a small area at a time. Over brushing leads to bubbles on the surface where the spar varnish is being applied.
Before applying a second or a third coat of spar varnish, sand the surface thoroughly. It should be wiped off with a tack cloth to remove any dirt, dust or particles before adding more varnish. Always let the varnish dry thoroughly in between coats.