Reply by Wildwood

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Posted on Spar Urethane - Minwax vs. Varathane

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2711 posts in 2611 days

#1 posted 11-18-2014 08:03 PM

Spar varnish simple mixture of;
Oils – Linseed Oil or Tung Oil
Resins – Alkyd, Phenolic, or Polyurethane
Solvents – Mineral Spirits, Naptha, or Paint Thinner

Your better grade Marine spar varnish contain Tung oil.

What Varnish Should I Use For My Job?
To determine what type of varnish you should use for a particular job, you must look at what type of oils and resins are contained within a varnish and what the ratio of oil to resin is. Varnishes that contain a larger amount of oil to resin are called long oil varnishes. Varnishes that contain a lower amount of oil are called medium oil varnishes. Long oil varnishes are more flexible than medium oil, but also softer. Medium oil varnishes are harder, but are more brittle. For exterior us e, a long oil varnish is best. Because it is more flexible, the varnish will expand and contract with the wood as changes in temperature and humidity take place. A medium oil varnish will not move as much and therefore as the wood moves and the varnish does not, the varnish will soon start to crack and peel. Medium oil varnishes are best used indoors where a lot of wood movement does not occur and a harder finish is desired. The resin contained in a varnish is also important in determining what type to use for your project. Some resins are more elastic than others, making them best suited for exterior uses. Phenolic resin is more elastic than other resins, therefore it will be able to withstand the extreme wood movement of exterior projects without quickly breaking down and cracking. Alkyd and polyurethanes are better suited for interior use. Not as important, but still a factor is what type of oil is used. Tung oil is more water resistant than linseed or other oils, therefore it would be a better oil for exterior use, but much more expensive.
Putting this all together, we basically come down to two categories, interior and exterior use. For exterior use, a modern spar varnish which is long oil and is made up of tung oil, Phenolic resins, solvents, dryers an Ultra Violet blockers (to protect the color of the wood from fading) is probably your best choice.

-- Bill

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