You’ve chosen your project with care, planned how to approach it, measured, cut, sanded, smoothed, stained, and applied a polyurethane finish. The piece looks great, you’re happy with your work but as the final coat dries and cures, you see it has a cloudy finish.
Oh, man, after all that work, and now the project seems ruined. You’re discouraged, and can’t look at it. But, the next day, you decide the best way to deal with a problem is through it, not around it, and are determined to find a solution, a fix that makes sense.
We’re glad you came here, because we believe we can help. Let’s take a look and see what we can come up with for you.
What is Polyurethane?
Technically speaking, polyurethane has two main chemical components: polymers and urethane. Thus, its name: poly (from polymers) and urethane from, well, urethane. The joining of these components forms a stable, safe, durable and often used product that is used as a wood finish.
It is heat-resistant, and comes in many forms. It is essentially a liquid plastic – a specific type of polymer. Other types of polymers include polyethylene, polypropylene, PVC (yes, the pipes), nylon, and teflon. For our purposes, we’ll be considering polyurethane only in its liquid form as it is used as a wood finish.
We have written of polyurethane in the past. It is basically a plastic applied to wood surfaces in its liquid form that dries to a solid plastic sheet. It is a film finish – it does not sink into wood. Instead, it sits on the surface and gives a waterproof coating to protect it. You’ll find that article on water-based polyurethane here.
There are also oil-based polyurethane products. Oil-based polyurethanes will last longer than water-based options, but will take 2-3 times longer to dry and cure. There is also an odor, and respirators are recommended when using it.
Water-based products will have lower volatile organic compounds (VOCs) than oil-based, which is an environmental advantage. But because of the lower VOC levels, it will not last as long as oil-based.
Many woodworkers, especially with flooring installations, prefer the oil-based polyurethane because it looks better, lasts longer, and is less expensive. But, again, we’ll be discussing water-based polyurethane in this piece to answer the main question.
We just wanted to give you a brief introduction to polyurethane first. Now, on to cloudy finishes.
What Causes a Cloudy Polyurethane Finish?
The finish you applied has turned cloudy and white, and you’re wondering why. Other finishes like lacquer and shellac can turn milky as a result of moisture that got trapped in them without the opportunity to evaporate during the quick-drying process. But it’s different with polyurethane.
A cloudy finish can be caused by the buildup of zinc oxide. Polyurethane dries to a naturally glossy finish. In order to create a satin or matte finish, manufacturers add zinc oxide as a flattening agent, which disperses reflected light and cuts the gloss from the sheen of that natural finish. The zinc oxide can collect at the bottom of the can.
So, the solution to prevent this cause of a cloudy finish is to stir well, and then stir well again.
Another reason you might have a cloudy finish has to do with where you stored it. If the can was subjected to extreme temperature fluctuations, a very hot or a very cold environment, the likely result will be a cloudy finish.
So, the solution is to avoid storage in places where extreme hot or cold conditions will affect the polyurethane in a bad way.
What To Do With That Cloudy Polyurethane Finish
Not to worry, no matter the cause. The cure is the same, and it’s not that difficult. Follow these steps:
Make sure the final coat is dry. Be patient here. Sometimes a polyurethane finish will appear a little milky as it is drying. If it’s still wet, let it dry to make sure there is a problem needing fixing. There may not be, and your patience will tell.
Your work area will need to be both clean and empty of a heat source. Actually, this is good advice for any finishing job.
Mineral spirits. Soak a clean cloth or finishing sponge with mineral spirits. Wipe the affected area down through the first coat; allow it to dry, soak the cloth or sponge again and repeat this process until you are down to the stain. Allow it to dry completely.
We’ve recently written about mineral spirits here. Check it out.
Sandpaper. Lightly sand the area with a fine-grit paper, 220-grit should be good, until all of the area is cloudy-free and all of the polyurethane has been removed. A fine steel wool could also be used, as well, but a 220-grit sandpaper will work just fine for you, too. Sand with the grain.
Clean. Make sure all dust is removed and the area is now fully clean. A slightly damp cloth can help. Any lingering dust will be seen through the finish, so think cleanliness is next to godliness.
Stir. Stir the polyurethane well, and then stir it again. Or, better yet, maybe a new can might be in order.
Apply. With a paint brush, apply new coats of finish, following the label directions on the can, allowing enough time for each coat to completely dry before applying the next. Be sure to stir well before applying each new coat.
If the entire surface of that new tabletop you built, or whatever your project might have been, has a cloudy appearance, then of course the entire surface will need to be treated in this same way. Allow it to dry, use mineral spirits to spirit away the polyurethane down to the stain, sand or steel wool the whole surface, clean it thoroughly, and reapply the entire finish coats.
Be sure to use a brush or sprayer to apply polyurethane, too. Rollers can be problematic, especially if you have chosen a wrong nap. Foam applicators work well and don’t leave brush marks, although sanding with a fine-grit paper between coats will get rid of any brush marks you might find.
In fact, sanding between coats, generally, is a good idea. Just remember to clean the surface thoroughly after cleaning. A finish of 2-3 coats should be sufficient for all projects.
In researching for this piece, we found some rather unusual methods for clearing up a cloudy polyurethane finish. One involved the use of a mayonnaise coating over the finish; another used a heat gun to draw out moisture just below the surface of a poly coat; a third used a hair dryer, again, to draw out and evaporate moisture.
While we don’t dismiss these methods as possibly effective, we’ll stick with the method we have outlined above.
Finally, search as we may, and after 10 videos, we were unable to find one to recommend.
This method of clearing away clouds in your finish is a long-established method that was used before heat guns and hair dryers. We’re sticking with it because we like the results.