Clear Outdoor FInishes

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This article is from Issue 58 of Woodcraft Magazine.

Weather will eventually win, but here’s how to play good defense.

By Joe Hurst-Wajszczuk

In truth, the great outdoors isn’t that great for wood. Sunlight (UV radiation), moisture, plus a host of insects and microbes are all dead-set on turning your project back into the dirt from whence it came. Paint provides the best defense; unfortunately, the pigments that protect also cover up the wood’s grain and your workmanship. If you want the wood–and your work–to shine through, you need a clear finish.

Choosing a clear finish these days isn’t as clear as it once was. In addition to a host of familiar offerings, advanced chemistry (influenced by tighter VOC restrictions) has created a bunch of new options. Choice can be a good thing, but with so many products silently sitting side-by-side on the store shelf, it’s easy to get confused.

To help you make sense of your options, I’ve divided outdoor clear finishes into two basic categories and provided examples to illustrate the range of products within each group. Additionally, I’ve included two “outside of the box” products that deserve consideration. 

To find the best defense for your next project, start by comparing the major differences, such as application ease and service life, and select a category. Next, check the attributes within each subcategory, and pick a winner. (To make quick sense of the facts, skip ahead to the chart on p. 50.)

This shouldn’t come as a surprise, but eventually, weather will win. That said, if you choose the right product and do what it takes to keep up its defenses, you can help your project hold out for many years. 

Varnishes

Pros:

  • Long setup time
  • Offers the best defense against moisture and UV radiation
  • Damage to finish can often be repaired without touching the wood

Cons:

  • Sanding is required between coats (with most)
  • Applying requisite number of coats can take days
  • If the finish fails, you’ll need to strip the piece and start from scratch

Whether the can says “spar” (varnishes are a traditional marine finish) or “exterior,” it’s safe to assume that the product is formulated to be flexible enough to withstand outdoor conditions and to protect against moisture and UV radiation.

Varnishes are intended to be built up to create thick physical barriers against moisture, UV radiation, and abrasion.

Alkyd (Oil-Based) Varnishes

Traditional oil-based varnishes lead the pack in durability. Most can hold their own for 2 to 3 years before they look chalky. At that point, reviving the finish involves scuff-sanding the topcoat and applying a fresh coat. (Note: Sanding isn’t needed with Waterlox. The topcoat bonds to the existing layers.) With regular inspections and reapplications, repairs can be made, almost indefinitely, without touching the wood. That said, don’t put off maintenance. If damage makes it down to the wood, you’ll need to completely strip the project and start from scratch.

With a top-quality natural brush, applying an oil-based varnish isn’t difficult, but it does take time. Factoring in the drying times and the requisite number of coats (from 3 to 8), it can take days to apply finish and a few more days before the project is ready for use.

Acrylic (Water-Based) Varnishes

As with most water-based finishes, acrylic varnishes look white when wet, dry quickly, and have no solvent smell. Unlike first-generation varnishes, the samples I tried flowed out smoothly and did not raise the grain. In terms of appearance, water-based varnishes are whiter than alkyds (see photo at right), but neither had the color-sapping blue cast of first-generation acrylics. The color clarity also allows you the ability to use this varnish as a protective topcoat on painted projects.

If you’re looking, look no further. Acrylics cure considerably faster than oil-based varnishes and quicker than some oils. (Using a painter’s pad, I could apply a smooth coat of acrylic in roughly half the time it took to brush out and tip off the same area with an alkyd.) 


The biggest surprise was how much easier acrylics were to sand, which sometimes tend to “corn up” when scuff-sanding between coats. If you find that you sand a lot to correct imperfect brush work, this may be the reason to try acrylic.

Whether acrylics resist cracking, peeling, or blistering better than alkyds remains to be seen, but based on these initial tests, there are plenty of reasons why you should give acrylics a try.

Oils

Pros:

  • Fast and easy application
  • No sanding required between coats
  • Finish will not chip or peel

Cons:

  • Thin barriers offer bare-bones protection against moisture and UV radiation
  • Most oils cannot be built up for additional protection
  • Frequent inspection and regular recoating required

If you’re looking for a “flow it on, let it soak, and then wipe it off” finish, you’re halfway there. As a group, oils are fast and easy to apply and provide the most “natural” feel and appearance. Unfortunately, in comparison to varnishes, these thin-skinned finishes don’t offer as much protection against Mother Nature. And because most oils cure so soft, you can’t expect multiple coats to create a thicker, more durable defense.

As a group, oils are the most easily misunderstood because the products have completely different makeups. To understand your choices, it helps to divide this category into subgroups: hardening, oil/varnish blends, and polymerized oils.


Hardening Oils

Tung oil is one of the few naturally-hardening oils that is suitable for outdoor use. To use, simply wipe it on, wait, and then wipe off, and wait (each coat requires a day or two to cure). After about five coats, tung oil offers a decent defense against moisture; however, because it lacks UV inhibitors, projects exposed to sunlight will eventually turn grey.

Made from a blend of natural oils (including linseed), Odie’s Oxi Oil is solvent-free and nontoxic. In contrast to slow-curing tung oil, Odie’s looks good after only one coat and dries in only 72 hours. To use, simply apply with a rag or abrasive pad, give the oil 30 minutes to sink into the wood, and then buff off the excess. Because it contains natural waxes and UV inhibitors, Odie’s has an edge over tung oil when it comes to combating moisture and sunlight, but the barrier is still thin. To keep defenses up, you’ll need to monitor your project and apply a fresh coat when the surface starts to appear dry.

Oil/Varnish Blends

In theory, oil/varnish blends combine the best of both worlds. The oil makes the finish easier to apply (with a brush, rag, or garden sprayer) than heavier-bodied varnishes. The resins help the finish cure faster and offer additional protection against moisture and abrasion. However, while blends offer a nice-looking finish, the thin barrier still doesn’t do much to block moisture or vapor. And while most blends contain UV inhibitors, not much defense can be packed into a clear coat that’s only a few microns thick.

If the piece spends most of its days outdoors, you should plan on making regular inspections and applying a fresh coat of finish as soon as the wood appears dull (about every year). The good news is that oil/varnish blends don’t flake. In most cases, a project might only require a light scrubbing or power-washing before it’s ready for recoating. Furthermore, repairs are easy. If the wood starts to stain or turn grey, you can usually get away with spot-sanding and then applying a few touch-up coats to the problem spot.




Polymerized Oils

Although not evident from the label, Tru-Oil is different enough to deserve a special category. This polymerized (“polymerization” refers to a cooking process in an oxygen-free environment) product is essentially partially-cured. Once exposed to oxygen, the oil cures harder, faster, and glossier than tung oil and most oil/varnish blends. With multiple coats, it’s possible to build up a decent film finish that can hold its own against the elements.

The downside to the polymerization process is that the curing clock is already ticking. Compared to other oils and blends, Tru-Oil tacks up quickly and may develop tiny cracks if it’s applied too thick. For these reasons, it should be reserved for smaller, more manageable finishing projects, such as knife scales and gun stocks.  

Looking at Linseed Oil? Look elsewhere.

Sometimes all-natural isn’t good enough. Although it’s used as an ingredient in some finishes, linseed oil (derived from the seeds of the flax plant) isn’t able to stand up to weather on its own. As a finish, it dries soft and is easily penetrated by water. In addition, because the oils can feed micro-organisms, oiled surfaces may turn black.

One-Time And Woodlife Classic

When shopping for clear wood finishes, I found two products that don’t fit either the oil or varnish categories. In the event that your next woodworking project might take you into new territory, consider these choices.

OneTime Wood Protector

This penetrating finish is a unique nontoxic acrylate resin blend that penetrates wood and then hardens to provide long-term protection. Originally intended for decks, OneTime can be applied with a brush, roller, or garden sprayer to any unfinished wood, including pressure-treated lumber. According to the manufacturer, the finish provides UV and moisture protection for seven years (most deck finishes last about two years). OneTime is well suited for a variety of outdoor projects ranging from arbors to picnic tables, but be sure that you apply the finish outdoors so that the resin gets the sunlight it needs to cure. OneTime is available in “natural” and five additional colors.

Woodlife Classic

If you’re looking for a finish that looks like it isn’t there or want additional protection before applying a topcoat, check out this 100% clear acrylic primer/preservative. With nearly double the fungicide of standard sealers or stains, Woodlife protects wood against mold and mildew. As a sealer, the product also prevents moisture-related damage such as swelling and checking. To apply, brush, roll, spray, or dip parts. (Note that because it does not contain UV-inhibitors, treated wood will eventually turn grey.)

Online Extra

For free additional weather-fighting advice, check out “Wood vs. Wild” from Woodcraft Magazine #22, April/May 2009. Go to woodcraftmagazine.com and click on Online Extras.

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