Did you know that polyurethane is one of a few reliably waterproof wood sealants. That’s because other wood finishes are more water-repellent than waterproof, thanks to their microporosity.
A microporous finish is one that keeps out water droplets, but water vapor can still pass through it. So, these types of wood finishes, (like Linseed oil or Tung oil), can be fairly easily water-damaged.
But, not so with polyurethane. This plastic-like finish can form a seal so water-tight, that certain poly’s can even be used on exterior wood.
So, in this post, you’ll find out what happens to tree bark crafts if you don’t seal them. You’ll also learn what the three main types of polyurethane finishes are.
And keep reading to discover what kind of exterior sealant you should use on outdoor tree bark decorations…
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What Happens If You Don’t Seal Tree Bark?
Well, tree bark in and of itself is surprisingly water resistant. And it has to be, since bark protects trees from drying out and/or from becoming saturated with rainwater.
However, once a log has been cut from a tree, it’ll have exposed end grain.
And that end grain will lose a lot of moisture very quickly if it is not sealed. And this is regardless of whether the rest of the log still has tree bark covering it.
This loss of moisture, (via the end grain), means a tree log can dry out and start to crack.
So What Can You Use To Seal Tree Bark?
A water-resistant urethane-based sealant should do in a pinch.
Urethane, (which happens to be the key ingredient in polyurethane wood finishes), will form a flexible film around tree bark. And this is all thanks to the microscopic resin molecules that make up this product.
Those resin molecules form a tight bond as polyurethane dries and cures. What’s more, urethane finishes leave behind a surface film with a bit of give to it.
Which is why these finishes are more scratch resistant than other sealants, such as varnish or lacquer.
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3 Reasons Why Woodworking Is So Goo...3 Reasons Why Woodworking Is So Good For YouAnd What Types Of Polyurethane Finishes Are There?
There are three main types of polyurethane wood finish that you can purchase:
This particular type of poly product will give wooden surfaces the most natural look. It is also the one that takes the longest to dry too, as it is a solvent-based finish.
This poly product is more milky in appearance, (inside the can), compared to oil-based poly’s. However, a clear water-based polyurethane sealant will dry and cure into a clear coat.
This polyurethane also cures the fastest, thanks to the quick-drying evaporation of the water inside it.
Water-Based Oil-Modified Polyurethane
This polyurethane finish blends the previous two poly finishes.
It contains less oil than oil-based poly, so it has lower VOC levels. VOC’s (Volatile Organic Compounds) are those eye-watering fumes that come off of solvent-based finishes and paints.
But, while this finish is partially water-based, it doesn’t dry as fast as a wholly water-based polyurethane.
So What Is The Best Polyurethane For Tree Bark?
An oil-based polyurethane finish is perfect for any tree bark wood that’ll be spending it’s time indoors.
But, when it comes to bumpy bark, getting a nice and even coat is going to be pretty difficult if you apply polyurethane using a brush or cloth.
So, for a truly even coat, make sure you use a polyurethane spray product. An aerosol spray will give tree bark the clear and all-over protective finish that it needs.
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What About Exterior Decorations? What Polyurethane Should You Use On Say A Tree Stump?
For complete protection from all-weather, you should use a Spar Urethane Aerosol product instead.
Spar Urethane finishes also contain urethane just like polyurethane finishes. In fact, these two wood sealers pretty much contain the same stuff — just in slightly different amounts.
But Why Use Spar Urethane Instead Of Just Regular Polyurethane?
It’s because Spar Urethane has additional ingredients added to it that make it much more suitable as an outdoor sealant. Which is why this particular finish is used to seal outdoor decking and boats.
Now, if a Spar Urethane finish sounds right for you, then you’ll need this wood sealant in aerosol form. And the best one on the market is Minwax Helmsman Spar Urethane Spray.
Minwax’s Spar Urethane can be used to seal both indoor and outdoor tree bark decorations. And this oil-based sealer has enough flexibility to it to allow wood to shift around — which can happen a lot due to changes in humidity.
This urethane finish is also a fair bit thicker than regular polyurethane. So you only need 1-2 coats of Spar Urethane (as opposed to polyurethanes 3+ coats).
But, once it has dried and cured, it’ll give your tree decorations incredible waterproofing.
To Wrap Up, Here Are The 3 Key Takeaways From This Post…
- 1). Tree bark is a naturally water-resistant material, protecting the wood underneath from drying out.
- 2). But tree logs will have exposed end grain. Moisture can escape from that end grain, causing logs to dry out and crack.
- 3). You can use a polyurethane aerosol spray to seal interior tree bark decorations. However, for exterior pieces, use a weatherproofing Spar Urethane sealing spray instead.
Yang, Han-Seung, et al. “Water absorption behavior and mechanical properties of lignocellulosic filler–polyolefin bio-composites.” Composite structures 72.4 (2006): 429-437.