Spar Varnish for Exterior Wood

¶ Spar Varnish. All wood surfaces exposed to the sun should be covered with spar varnish if they are to be varnished rather than painted. And no filler should be used. The filler decomposes and crumbles under the action of the sun and temperature extremes. Build up the surface with several coats of varnish, sand each lightly, clean up and flow on the last coat freely.

¶ As the name indicates spar varnish was first made to protect the spars of ships by keeping out the moisture, and of course it had to be a varnish which would resist both moisture and the salt air at sea. Today spar varnish includes all long-oil varnishes of a similar type and for similar purposes. The name spar is also used to designate a group of very tough and elastic interior varnishes.

¶ As has been said spar is a long-oil varnish, which means that it contains a much larger proportion of oil to its gum resin content than other varnishes. Spar is dark in color, does not dry rapidly, is very elastic and retains this quality for a very long time, has only a moderate gloss and is very durable. Spar is still used largely on ships, but also on many exterior surfaces wherever a tough, elastic weather and water resistant varnish is needed especially for wood surfaces, but also on metals.

¶ Considering the characteristics of this varnish further, it is interesting to note the specification for it published in Circular 103 of the Bureau of Standards, U.S. Government. The circular reads, in part: "The varnish shall be suitable for use on both outside and inside surfaces of vessels, buildings, etc., and must be resistant to air, light and water. The manufacturer is given wide latitude in the selection of raw materials and processes of manufacture, so that he may produce a varnish of the highest quality. It must meet the following requirements;

¶ Appearance - Clear and transparent.

¶ Color - Not darker than a solution of 3 g. of potassium dichromate in 100 cc of pure sulphuric acid, specific gravity 1.84.

¶ Flash Point - (Closed cup) not below 30 degrees C (85 degrees F).

¶ Non-volatile Matter - Not less than 40 per cent by weight.

¶ Set to Touch - In not more than 5 hours.

¶ Dry, Hard and Tough - In not more than 24 hours.

¶ Working Properties - Varnish must have good brushing, flowing, covering and leveling properties.

¶ Safety of Working - Varnish must pass the draft test.

¶ Water Resistance - Dried film must withstand cold water for 18 hours and boiling water for 15 minutes without whitening or dulling.

¶ Toughness - Varnish must pass a 50 per cent Kauri reduction test at 24 degrees C. (75 degrees F).

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