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.
46- Desert Outdoor Finish
High Resolution Version
Outdoor finishes can be one of the trickiest types of wood finishes to master. Why? Because there is never a one size fits all solution. As woodworkers, we are used to dealing with the seasonal wood movement of our interior pieces. But when one of our creations is destined to live outside, there is much more to consider. Depending on your region’s climate, your project could experience anything from snow and ice to hot desert sand storms. How the heck do we prepare for that?!?! Well, it is still an area that I am learning much about. I haven’t built many outdoor pieces and most of my knowledge comes from my reading and conversations with other woodworkers. Here in the Arizona desert, we have mild winters, occasional isolated rain spells, monsoon season (a temporary time of increased humidity), frequent dust storms, and of course, blazing hot summers where my shop can reach 125F, if left to its own devices. So in these harsh conditions, how would you finish your solid alder front door? For me, the answer lies in something most of us use on a daily basis: our workbenches. Much like your workbench, an Arizona front door will receive a great deal of punishment. And when an item is going to see that much abuse, sometimes the focus has to shift from absolute protection (a losing battle), to repairability. And that’s where my special homemade oil/marine varnish blend comes in handy. Watch the video and see what you think.
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