Difference Between Spar Varnish and Regular Varnish? - Question of the Week
This week’s question comes from K Sanchez who writes: “Please tell me the difference between spar varnish and regular varnish?”
Now some of you may have noticed that I posted this question while I was on the road last week. An inquiry from reader Kip made me realize that I did a pretty crappy job of giving the level of detail required to answer this question accurately. So, here is a modified answer to the original question:
All varnish contains a few basic components: oil, resin, and a solvent. By modifying the type and amount of oil and resin, we can get a bunch of formulations for both indoor and outdoor use. Our oil options include linseed oil, tung oil, and other less expensive synthetic oils. Common resins include alkyd, phenolic and polyurethane.
So before we dig deeper, let’s talk about why we need a different varnish for outdoor applications. Any wood stored outside is going to be exposed to a wide range of temperatures and weather, as well as a good dose of damaging UV rays. The changes in humidity cause the wood to expand and contract, and a standard indoor finish would simply crack and deteriorate under these conditions. Spar varnishes are typically designed to give us general protection, flexibility, and UV protection.
The ratio of oil to resin has a dramatic effect on the way the varnish will behave. For instance, using a small amount of oil and a large amount of resin will produce a very hard and brittle finish. Obviously, this is not suitable for outdoor applications. So what makes more sense is to create what is known as a “long-oil varnish”, that is, a formulation that contains a greater percentage of oil. The extra oil results in a softer, more flexible finish. And from what I’ve seen, tung oil is generally considered to be the best oil for these outdoor applications.
As far as the resins go, it seems to be a mixed bag when it comes to spar varnish. While many articles claim that phenolic resins are the best and most common, I found that alkyd resins tend to be prominent as well. Both Helmsman Spar Urethane and Epifanes contain alkyd resins. Its interesting to note that they also both contain some urethane resins to “fortify” the finish.
Most spar/marine varnishes will contain other important additives such as UV blockers, that give the wood that extra bit of protection it needs in harsh conditions. So generally-speaking, my preferred outdoor varnish would be a long-oil varnish containing tung oil, alkyd or phenolic resins, and UV inhibitors.
Related Posts
- April 14, 2008 — Episode 46- Desert Outdoor Finish
- September 21, 2008 — Danish Oil on top of Shellac? - Question of the Week
- September 8, 2008 — Water-Borne vs. Water-Based - Question of the Week
no comments yet.
