A commissioned project for a wedding gift. Designed as a outdoor courtship style bench. Laminated cedar glued w/ titebond III.

Here’s a table that didn’t like the weather. It was a quick build to fill a pressing need. My daughter and her husband had moved to Phoenix and did not have a dining table of any sort. On our first trip to visit them, I went to Home Depot, bought a Ryobi $100 table saw, router and drill. Then a laminated sheet of pine, a few two by fours and some four by fours.

It took a few hours to build, then stain and brush on poly. A couple days later it was in their kitchen and was ready for use. For about a year it served it purpose. When they moved to a different house, they had acquired a dining room set and the table was put out on the patio where it looked quite nice by the way.

However, the sun and the heat of Arizona summers took their toll. A year and a half later, the table was unrecognizable and had surrendered it’s stability to the forces of nature. First the finish yellowed and cracked and finally the wood just had to move. And move it did. The top was warped and twisted beyond reason and the poor table died a horrible disfiguring death. It now stands as a testament to the slow but sure supremacy of nature.

Why We Need Outdoor Formulations

Before we dig into the details, 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. These elements serve to break down the finish over time. Furthermore, 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 not only protect the wood, but also give it the flexibility and UV protection it needs to last for years. And the name “spar varnish” comes from the boating world, where the long wooden poles that support the sails are know as spars. So a spar varnish needs to be one that can withstand the rigorous conditions of seafaring life.

The Components

Nearly all modern varnish contains a few basic components: oil, resin, and a solvent. By modifying the types and amounts of these components, we can create a whole range of mixtures that vary in price and are specifically suited for either indoor or outdoor use. Fortunately for the inquisitive finisher, there are only so many ingredients that manufacturers have to choose from. And this makes it easier to see beyond the marketing jargon to deduce how a particular finish will behave. Here are the most common recipe ingredients:

Oils – Linseed Oil or Tung Oil
Resins – Alkyd, Phenolic, or Polyurethane
Solvents – Mineral Spirits, Naptha, or Paint Thinner

Oil to Resin Ratio

When a varnish is made, the ratio of oil to resin can have 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 but somewhat brittle finish. Obviously, this is not suitable for outdoor applications since we need an outdoor finish to be flexible. 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 that will not crack when the wood expands and contracts.

Oil Types

The most common oil used to make varnish is linseed oil. Its lower cost makes it the most practical choice for both indoor and outdoor formulations. But many believe that tung oil is actually better for outdoor use. After all, a higher quality oil should equate to a higher quality varnish, and thus a higher price tag. As a result, many of the high-end marine varnishes will be made with tung oil instead of linseed oil.

Resin Types

Generally speaking, phenolic resins are best-suited for outdoor use. But that doesn’t mean every spar varnish is made with phenolic resins. Much like the situation with oils, the better product is also the most expensive. So you’ll find plenty of outdoor formulations using alkyd and urethane resins. A popular finish like Helmsman Spar Urethane contains urethane modified alkyd resins. A higher quality finish like Epifanes contains phenolic modified alkyd resins. There are many other brands of outdoor oil-based varnish, but the ingredients list is usually much more revealing than the words on the front of the can.

Sun Block For Wood

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. UV light will not only damage the wood, but also the finish itself, eventually resulting in finish failure. So its a good idea to use a finish containing UV-blockers for any outdoor project.

Conclusion

Generally-speaking, my preferred outdoor varnish would be a long-oil varnish made with tung oil and at least some phenolic resins and UV inhibitors. And most times the brand I reach for is Epifanes.

If you’d like to learn a little more about how I apply varnish, check out this article: A Better Way to Apply Spar Urethane

If you want to learn more about my favorite finishing method, you should check out my DVD, A Simple Varnish Finish. My goal with the DVD was to demystify finishing by going over each and every step of the wiping varnish process. Even someone new to the world of finishing can create a show-stopping finish if they follow the methods outlined in the DVD. So check it out!

The product I recommend using in the video is Epifanes Marine Varnish.

“Quick question on joining treated pine – I would like to face glue surfaced 2×6 treated pine boards. Does the fact that i am using treated lumber reduce the strength of the joint or make the glue less effective somehow? This is for an application that will have to be waterproof.”

My reply is actually an excerpt from an article I found a while back from Iowa State Univeristy. Full Article Here.
Deposits on the surface of treated wood present problems in gluing. Oil-type preservatives typically present more problems than waterborne treatments. Wood treated with very high retentions of creosote or pentachlorophenol in heavy solvents is essentially non-gluable. Low retentions of penta in light solvents may be somewhat more readily glued than wood treated with creosote. Wood treated with waterborne chemicals usually can be glued without major problems, if properly dried. Planing or sanding the surface before gluing is recommended to enhance bonding. Select the adhesive appropriate for the exposure condition among conventional wood adhesives. Only resorcinol resin glues provide completely waterproof gluelines with wood; urea resin glues and polyvinyl resin adhesives generate only modest water-resistant glue bonds. Some types of elastomeric construction adhesives offer good resistance to moisture and are much more tolerant of high wood moisture contents and low temperatures than conventional wood adhesives.

If anyone has anything to add, please do so in the comments section. I personally have very little experience gluing treated wood.

“I am using Helmsman Spar Urethane to refinish the front door of our house. It’s a one piece flat door — no recessed panels. I am applying it with a quality brush but I still get brush marks and hi-low points, also tried a sponge brush with same results. I know it’s barely noticeable but I absolutely abhor brush marks. After 4 coats, I have managed to sand it down fairly flat. Is it possible to thin Helmsman and spray it with an HVLP gun? There is a Helmsman spray can available but I have so much Urethane left over. To thin or not to thin?”

The Power of Dilution

Polyurethane, whether its an outdoor or indoor formula, takes a little finesse to apply without bubbles or brush strokes. You need to use the right brush and it takes a little practice to get the technique right. Sponge brushes are certainly another option, but the results are not always a significant improvement. Most of the trouble comes from the fact that the urethane solution is so thick. It can be like trying to lay down an even coat of corn syrup. So one way to significantly improve the workability of your urethane is to thin it with mineral spirits. Thinning by about 10-20% will make brushing significantly easier. And thinning even further would allow you to spray via HVLP, but that’s definitely not my favorite way to apply an oil-based varnish. Thinning allows the finish to self-level a little better, and ultimately that will help you avoid brush marks. The only disadvantage is you need to apply more coats to get the desired film thickness you are after. But if the payoff is a smooth finish with no brush strokes, who cares, right?

The Wiping Method

Now there is yet another way to apply polyurethane that I feel is the easiest, beginner-friendly method. And frankly, its the method I still use. It involves abandoning brushes all together. Here’s what you do. Thin the varnish 50% with mineral spirits, paint thinner, or naptha. Then get yourself a clean cotton rag (old t-shirt material works great), and fold it into a nice square applicator pad. I like it to be palm-sized. Wet the applicator with your thinner and then dip it into your diluted finish. Wipe on smooth thin coats, overlapping the previous stroke with each pass by about half. This method will result in a super smooth finish with no bubbles and no brush strokes. But remember, you will need to apply twice as many coats to get to your desired film thickness. Because you are actually applying less material to the surface, the dry time is significantly decreased as well. So in some climates (hot and dry), you’ll be able to apply as many as three coats per day. Just remember that when you use a wipe-on urethane, you aren’t trying to flooding on a real thick layer. You are just coating the surface evenly with a thin wet film.

If you want to learn more about my particular finishing method, you should check out my DVD, A Simple Varnish Finish. My goal with the DVD was to demystify finishing by going over each and every step of the wiping varnish process. Even someone new to the world of finishing can create a show-stopping finish if they follow the methods outlined in the DVD. So check it out! And for those that are wondering what my preferred outdoor varnish is, its Epifanes Marine Varnish.