Difference Between Spar Varnish and Regular Varnish? – Viewer Question
This week’s question comes from K Sanchez who writes: “What is the 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. A finish like Helmsman Spar Urethane contains urethane modified alkyd resins. A higher quality finish like Epifanes contains phenolic modified alkyd resins and a tung oil base. Since phenolic resins are considered more durable and better suited for outdoor conditions, Epifanes would be my finish of choice outdoors. There are many other brands of outdoor oil-based varnish, but the ingredients list is really all we need to determine an approximate level of quality.
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, at least some phenolic resin, and UV inhibitors.
Stinky Dresser! – Question of the Week
This week’s second question comes from Melanie. She writes:
My Father in Law made a gorgeous dresser for our son…problem is when we use it, the clothes smell like finish and then so does our baby. We’ve aired it out numerous times and it also aired in his shop before it came over. I’ve tried a couple of odor discs and sachets, but the smell comes back. I’m afraid to try anything stronger for fear of adding/combining the smell. FIL and my grandfather (who also does wood items) are both stumped. What can I use to safely conquer the smell and use the dresser?!
And here was my reply:
“Hey Melanie. This is actually a very common problem. As oil cures, it maintains an odor. Fortunately this odor dissipates over time. But on the inside of a piece of furniture, where very little air exchange takes place, the oil can produce an odor for many many years. So what can you do? The first option, which I’m sure is not feasible, would be to let the unit air out for a very long time. The second option would be to coat the interior with a new finish that doesn’t produce an odor. There are two finishes that are perfect for this: shellac and water-based polyurethane. One or two coats on the interior should do the trick. This material will lock in the oil odor while producing no smell of its own. Your FIL or Grandfather should be able to help you apply either material. So as an FYI for your woodworking relatives, its a good rule of thumb to leave the interior of casework (especially those intended for clothing or blankets) unfinished, or finished with a few coats of shellac or water based polyurethane. That should take care of it.”
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