Spar Urethane is a mold-resistant, water-resistant — and even UV-resistant — wood sealant.
Pure Tung oil, on the other hand, is fairly water-repellent. But, it does not include any of the other weatherproofing qualities of Spar Urethane.
That’s why Tung oil is far from a long-lasting exterior wood finish. But will a few coats of Spar Urethane help to make that wood-enhancing Tung oil finish more durable?
Well, in this post you will learn why Tung oil isn’t that water-resistant. You will also find out if tacky Tung oil can be sealed with Spar Urethane.
And keep reading to discover the 4 things you can do to make sure Spar Urethane adheres to Tung oil.
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Do You Really Need To Seal Wood After Applying Tung Oil?
Well, Tung oil can be used as a sealer. Problem is, it isn’t a very durable wood finish.
This microporous wood finish isn’t waterproof — although it is fairly water repellent.
This means that it will stop water droplets from sinking past its coat. However, water vapor and humidity can easily make their way past Tung oil.
On top of that, if you were to use Tung oil on exterior wood, it would need to be reapplied frequently. At least once every 6-12 months depending on how humid the local climate is.
In short, pure Tung oil is more of a interior wood finish, than an exterior wood sealer. And if you want to make Tung oil more durable, then it needs a weatherproof sealer over it.
Related Post: Will Applying Tung Oil On Exterior Pressure Treated Wood Protect It?
And What’s The Best Kind Of Exterior Finish You Can Put Over Tung Oil?
Spar Urethane, (also sometimes known as Yacht Varnish), is the perfect outdoor water-resistant sealer for Tung oil. Mainly used as an exterior sealant, Spar Urethane has been applied onto everything from boat decking to garden furniture.
And, thanks to added mold-resistant additives and UV-inhibitors, this low-maintenance sealer can reliably weatherproof a coat of Tung oil. And it only needs reapplying once every five years.
Now, what makes Spar Urethane a better sealer than polyurethane, (which is another urethane-based sealer), is its flexibility.
Thanks to the unique balance of ingredients in Spar Urethane (compared to polyurethane) this exterior sealer has more flex to it. And that in turn allows it to be more scratch-resistant than polyurethane sealers. Which is a huge advantage, because Tung oils lack of flexibility makes it prone to scratches.
Related Post: Is Tung Oil A Good Enough Finish For Outdoor Furniture?
Can You Really Use Spar Urethane Over Tung Oil? Here’s Our 4 Top Application Tips:
First off, Spar Urethane absolutely will stay on top of a coat of Tung oil. However, there are four key caveats to making sure this happens:
1). Use An Oil-Based Spar Urethane
As a general rule of thumb, when it comes to wood finishing, you should match oil finishes with oil-based sealers. And water-based finishes should be paired with water-based sealers.
This is because incompatibility issues can prevent two opposing wood finishes/sealers from adhering to each other.
Still, there are ways to get two incompatible finishes to work with one another, of course. For example, a thin coating bonding primer — such as Dewaxed shellac — can ‘glue’ two incompatible finishes together.
But, there is no need for any such bonding primer in this instance. Stick to applying oil-based Spar Urethane over a Tung oil finish, and you’ll be good to go.
2). Sand The Surface Before Application
You should always lightly sand smooth surfaces, (with high 120-grit sandpaper), before a coat of Spar Urethane. This is so that you create a bit of grip for Spar Urethane to cling to.
This is why we sand between coats of Spar Urethane too. And it is just as important a preparatory step when applying Spar Urethane over Tung oil.
3). Use A Spray-On Spar Urethane
After Tung oil wholly cures, it will have formed a solid seal on the surface of wood. And this seal won’t let water droplets — let alone another wood finish — sink past it.
So you won’t be able to apply a penetrating Spar Urethane over Tung oil. The Spar Urethane won’t be able to sink into the wood underneath the Tung oil coat.
But, a Spray-on Spar Urethane doesn’t need to sink into wood to form a water-resistant seal.
4). Tung Oil Must Cure First (Before You Apply Spar Urethane)
This is incredibly important, as Tung oil is a very slow drying and slow curing wood finish.
Now, first off, drying and curing are not one and the same. They are in fact two different processes that Tung oil wood finishes go through.
A dry Tung oil is one that has gone through a process of evaporation. This turns it from a liquid oil into a film coat on the surface of wood. And it may even feel solid to the touch.
A cured Tung oil is one that has gone through a process of chemical reaction. And, this turns it from a film into a solid durable resinous coat.
Now, curing takes an incredibly long time when it comes to Pure Tung oil — up to 30 days (or longer in humid weather).
But, until Tung oil has wholly cured, it is not ready for a top coating sealant. If you were to apply Spar Urethane onto a still curing Tung oil, that Spar Urethane will peel off.
Can You Put A Water-Based Spar Urethane Over Tung Oil? Oil-based Spar Urethane will adhere much better. If you use water-based Spar urethane, then use Dewaxed shellac as a bonding primer. Dewaxed shellac will allow a water-based Spray-on Spar Urethane to stick to a cured Tung oil coat.
So Does That Mean Tacky Tung Oil Can’t Be Sealed? Or Can You Get Spar Urethane To Stick To It…
If you want Spar Urethane to stick to any surface, that surface needs to be clean. So, it cannot have any liquid oils or wet solvents on it.
Now, a cured Tung oil coat is as clean a surface as any (when it comes to applying Spar Urethane).
But, if Tung oil is still tacky, that means it has NOT cured. But, whats more, it means that it may still be leaching oils.
In fact, if you were to swipe of your finger across tacky Tung oil, it’ll feel greasy.
On top of that, tacky Tung oil is likely still releasing solvent fumes (referred to as off-gassing).
Those fumes — or rather VOCs (Volatile Organic Compounds) — can become trapped underneath Spar Urethane. And that can result in trapped air bubbles inside the Spar Urethane finish.
Related Post: What’s With All These Bubbles In My Spar Urethane Finish?! [What You Need To Know]
To Wrap Up, Here Are The 3 Key Takeaways From This Post…
- 1). Spar Urethane needs to be applied onto wooden surfaces that do not have greasy oils or other chemicals contaminating it.
- 2). Tung oil must dry and cure before you apply Spar Urethane over it. It can take pure Tung oil up to 30 days to cure.
- 3). Use a Spray-on oil-based Spar Urethane over Tung oil for best results.
Arminger, Benjamin, et al. “On the drying behavior of natural oils used for solid wood finishing.” Progress in Organic Coatings 148 (2020): 105831.