Spar urethane is generally preferred for protecting exterior wooden surfaces exposed to sunlight, moisture, and temperature fluctuations. Spar urethane thrives at such protective duties due to its notable composition of UV blockers. But despite its reputation for withstanding moisture and sunlight, spar urethane has its problems. 

Spar urethane doesn’t appropriately blend with oil-based paints. Particularly, when mixed with paints containing polyurethane, spar varnish creates an uneven color tone, with poor consistency in the finish. Being thicker, spar urethane can also take a while to dry. Lastly, spar urethane can be hazardous, emitting notoriously strong fumes.

This article will explore the prevalent challenges associated with using spar urethane. Furthermore, this article will espouse remedial actions you can take to limit the handicap in each situation.

Let’s get the ball rolling.

1. Spar urethane does not properly blend with oil-based paints

This is by far the most prominent challenge with spar urethane. Due to its unique chemical composition, spar varnish is selective of the paints they mix with. Pigments based on oil don’t work with spar varnish. 

This challenge is even aggravated when you attempt blending spar urethane with paints containing polyurethane. 

We admit there is not much you can do to dramatically improve the consistency or color balance when mixing spar urethane with such paints. 

That said, thinning your spar urethane with mineral spirits is one clever hack to make your spar varnish more workable and integrable with other paints. This improves the ability of the finish to level on its own.

Thinning can be immensely beneficial when you intend to apply your spar urethane by brush. With such thinning, the propensity to leave brush marks on your finish is also reduced.

2. Spar urethane doesn’t dry early enough 

This is another almost-unavoidable challenge that comes with using spar urethane. Since spar urethane is inherently thicker than your traditional varnish, it doesn’t dry as quickly.

This means you would have to wait longer for it to get hard between successive coats. If you don’t wait long enough, the chances are high that soft spots would develop in your work, necessitating a resanding exercise.

A prominent determinant of how long spar varnish takes to dry is the temperature around the surface. If you are not in a humid climate, it could take a full day for a coat to dry before applying the next layer. In humid climes, you may have to wait up to two days.

If you are recoating, it is advised to aim for a relative humidity of 50% with a temperature around 70°F. If these atmospheric conditions can be created and sustained, your spar urethane can dry enough for you to touch within 120 minutes.

But this is still too early to apply the following coat. To add successive coats, you will have to wait up to 8 hours. 

Within 24 hours, you could be deploying the finished coat for minimal/light use. If you would be comprehensively using that surface, we advise you to wait up to three days. 

3. Spar Urethane not sticking

We have come across many woodworkers complaining of inadequate adhesion when finishing with spar urethane. While spar urethane doesn’t boast the best adhesive capacities, a lot of the poor sticking can be attributed to insufficient surface preparation before applying the finish. 

Before applying spar varnish, we recommend sanding the surface with 150-grit sandpaper. Ensure to keep the surface clean enough, removing the previous finish and scrubbing off all grease, wax, and paint.

This improves the surface’s ability to hold on to the coat. 

Such preparatory exercise is beneficial when applying your spar urethane to woods with remarkably tight molecular structures. 

For such wood, your spar varnish would ordinarily struggle to permeate the grain, meaning it wouldn’t stick well enough.

When you are done preparing your surface, you can – with a brush – apply your spar urethane, specifically a thin coat, at the start. 

Subsequent coat application should also be preceded by surface preparation. But this should only be after allowing the previous coat to dry for no less than four hours.

If that is the first time you are applying a coat to that wooden surface, three coats would do. Note that no sanding is required for the final coat.

4. Spar urethane produces strong fumes

If you have used spar urethane before, you can admit to how discomfiting the fumes can get. It could even worsen depending on the spar urethane brand you use. 

Well, if you manageably get a good brand, your spar urethane finish could still be food-safe if you executed the curing correctly. 

For the curing procedure, it is recommended that you live it alone for at least a month at a temperature margin of 65°F and 75°F. There are spar urethane brands that don’t take that long to cure. Some cure within two weeks.

Having established that, considering the possible noxiousness of the fumes from your finish, we advise that you only apply your spar urethane in a worksite that is adequately ventilated.

It would help if you also had some safety accouterment when applying your spar urethane. Always be kitted with a mask that protects your mouth and nose.

5. Spar urethane yellowing

We concede this isn’t the most prominent issue with spar urethane, but it still happens. Being an oil-based finish, it is not uncommon that surfaces with spar varnish would develop a yellowish hue with time. In some cases, the cover takes an orange-like tan.

The UV blockers will not permanently shut out the sun’s radiation (with the same efficiency) forever. With prolonged exposure to the sun, your spar urethane could yellow. 

Indeed, how quickly they yellow varies across brands. Yes, you don’t readily see varnishes based on water. 

If you notice that your spar urethane finish is significantly yellowing, you can enhance its look by recoating.

This is as straightforward as scuffing the yellowing finish of your sandpaper. 220-grit sandpaper or even 150-grit sandpaper would get the scuffing job easily done.  

When done, you can apply a new coat. Ensure that the brush you use is furnished with a natural bristle. Still, on the brush, a lot of your varnish yellowing can be traced to brush contamination.

Spare some effort to ensure any brush you are using is thoroughly cleaned of left-over particles from the previous project.

Related Article: 7 Problems With Valspar Paint (Explained for Beginners)

6. Spar urethane is expensive

Should we be counting the expensiveness of spar urethane as a “problem”? Sure we could when a quarter of spar urethane goes for a premium of $31 (depending on the brand). 

For comparison, a similar quart of polyurethane goes way lower at $14.50. That said, not all spar urethane finishes are too expensive. 

Brands like Rust-Oleum come a bit expensive (and you wouldn’t blame them too much given the prestige they command and unquestionable quality). 

But if you are on a tight budget, you can go for relatively cheaper spar urethane brands like Minwax® Helmsman®. The latter sometimes goes for half the price of Rust-Oleum.

While we have extensively touched on spar urethane’s problems, it is critical to emphasize in particular situations – like outdoor finishes – spar urethane counts among your best varnish options. 

Very few finishes can measure up with the resilience spar urethane gives when directly and sustainably exposed to atmospheric elements.

Related Article: 5 Best Deck Paint: Reviews And Buying Tips

Resources

What is Spar Urethane and how do you use it

What is Spar Urethane

How to thin and apply spar urethane correctly

Difference between spar urethane and polyurethane

Polyurethane vs Spar Urethane

Helmsman Spar Urethane not drying