White spots on polyurethane: How to fix them

Sometimes when the polyurethane dries on your project, it leaves white spots on the finish, marring otherwise nice-looking work. Maybe that only happens to me, but I don’t think so. Fortunately you can fix white spots on polyurethane. And when you know what causes them, you can prevent them in the first place.

White spots on polyurethane are generally caused by two things: Not stirring thoroughly enough before applying it, or a buildup of moisture. While preventing the white spots is easier than fixing them, you can fix them by sanding down the high spots and then drawing out the moisture. If the spot is small, you can touch it up with a furniture marker.

Fixing white spots on polyurethane

white spots on polyurethane
White spots on polyurethane can be caused by a number of things, but they are preventable, and small spots like this are easy to fix.

Rather than lecture you on how to prevent the problem first, I’m sure you’re here because you already have the problem. It’s happened to me. And when you’ve just spent a three-day holiday weekend on a project, you probably don’t want to think about stripping and refinishing part of it already. At least I don’t.

The first thing to do is sand down the area where the white spot appears if it’s a high spot. Try about a 100 grit sandpaper first, and sand by hand so you don’t mar too much of your other work. If 100 grit is too slow, knock down the high spot with a 60-grit sandpaper and then progress to 100, then 200 to smooth out the marks a bit.

If you’re lucky, the white spot is high enough on the finish that you can just sand it out. But it seems like it usually doesn’t work out that way.

The mayonnaise trick

I can’t say I’ve had much success with this but it doesn’t cost much so it’s worth trying. Try smearing some mayonnaise on the white spot and let it sit overnight. Mayo can draw moisture out from below the surface. Since the white spot is caused by excess moisture, the mayo will make it disappear, if it can draw out enough.

Touch-up for small white spots on polyurethane

If the mayonnaise doesn’t make the white spot go away, not all hope is lost. Get a set of furniture repair markers. The markers will be labeled with the wood they most closely resemble. Sometimes I find I have to go a bit darker than what the label says. A lighter-colored shade may not cover the white, but a dab of walnut is a lot less noticeable than white.

After applying the marker, I let it dry for an hour or so and then apply a thin coat of polyurethane over it to protect it. It doesn’t look perfect, but I may be the only one who knows I did it.

During my last project I groused that it seems like I spend more on wood filler and markers than I do on lumber. That may or may not be the case, but the filler and markers let me deal with mistakes much more quickly and easily, and the supplies do generally last more than one project. And even if your projects end up not costing much less than particle board furniture from the big-box store, they look better and they’re more durable.

If I had a lot of really big white spots, I would be reluctant to try this. But for a quick fix on small areas, say, the size of a quarter or smaller, why not?

What causes white spots on polyurethane

white spots on polyurethane
This drip probably would have turned into a white spot if I hadn’t brushed it out.

Several things can cause white spots on polyurethane. All of them are preventable, though some are easier than others.

Stir the polyurethane before use

One reason polyurethane can get white spots is due to a buildup of too much flattening agent, usually zinc oxide. The solution is to stir it thoroughly. It’s an honest mistake. It never occurred to me that you need to stir polyurethane like you need to stir paint, but it makes sense after you think about it.

Since I learned this, I started stirring my polyurethane before every use and it’s improved my results.

Alternatively, I guess I could switch to gloss, but I prefer how satin looks overall.

Too much stain or not enough dry time

Another reason it can happen is applying too much stain or not letting the stain dry enough. Apply a thin coat of stain, then wipe the stain off within an hour or so. Some brands of stain take longer to dry than others. I found Varathane stains dry much faster than Minwax. If you’re not satisfied with the finish you’re getting, using pre-stain wood conditioner first is usually a better answer than applying several coats of stain.

I’ve never run into compatibility issues, but since I always buy my stain and polyurethane at the same store that happens to be three minutes from me, and that store only carries one brand, I guess that isn’t too surprising. If you’re using a combination you’ve never tried before, it pays off to test on a piece of scrap wood first, before you devote a lot of time to a project.

Too thick of a coat

I got in a hurry in one project, and I glommed a couple of thick coats of polyurethane onto it, hoping to get it done faster. It didn’t work out very well for me. Varathane water-based polyurethane advertises itself as being a quick-dry formula. I find it dries enough for another coat in 15-20 minutes. That’s pretty quick. A larger number of thin coats looks better than a couple of really thick coats and it will probably dry faster in the long run.

Drips

I typically get white spots as a result of drips. Applying the polyurethane in thin coats helps to prevent this. But sometimes you end up applying a thicker coat than you intend.

To prevent this, I found it’s best to walk away after a coat of polyurethane, then come back 5-10 minutes later and look at the edges. If I see any drips accumulating, I run my brush along the area again, without adding any additional polyurethane (that’s important), to even it out. If I see any pools of white on larger surfaces, I may brush those out too, as that suggest I may not have gotten my application even. But those seem to recover better on their own in my experience than edges do.

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