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What is Really Causing White Line Syndrome?

By Kim M. Wahlgren
June/July 2011
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The wood flooring business doesn’t have much in the way of conspiracy theories, but one finish problem is approaching that status. Rumors and buzz about it—and its causes—spread rapidly by word of mouth and even faster in the blogosphere. It’s discussed so much, with so much wrath, that it has even earned a name that sounds like a disease: white line syndrome, or WLS for short.

The name is casually used any time white lines show up between boards in a wood floor. The reason it has earned its own moniker is that it seems to be so much more prevalent in recent years, particularly about the last five years. Some contractors who have never had issues with white lines before report suddenly having to redo floors, even though they’re doing everything the exact same way—with the same products they always have used.

This has all led to suspicions directed at the finish manufacturers: They’re changing product, using cheaper ingredients, messing up the finishes to comply with VOC laws, etc., etc. Contractors feel they are being thrown under the proverbial bus. But what really causes WLS? Is it truly a mysterious, misunderstood phenomenon lurking on your next job?

Types of WLS

The first question is: What, exactly, is WLS?

Not all finish experts agree on the terminology. Some use “WLS” to describe any time any sort of white lines show up between boards. Others make a distinction between what they consider to be “real” WLS—finish stretching between boards—and white lines that result from peeling finish. They do agree on this: Whatever you call it, the vast majority of calls they receive from customers saying they have WLS are actually about finish peeling at board edges.

Peeling Problems

Peeling described as WLS manifests itself along board edges, usually with oil-modified urethane (OMU) finishes. As the finish begins to lose adhesion and peel, it turns white in an irregular pattern that roughly follows the board edges (but isn’t an exact straight line) and often overlaps the top of the board slightly.

Why does peeling show up in this pattern? One answer has to do with abrasion, or lack thereof. OMU typically doesn’t stick well to itself—it needs a good mechanical scratch, or abrasion, in order to bond well with the next finish coat. If the floor isn’t abraded well enough between coats—as often happens when you’re afraid of creating swirl marks or cutting through the finish to bare wood—the finish won’t bond well, especially along lower-lying areas of the floor. (As one finish manufacturer put it, “It’s stuck, but it’s not stuck well enough!”)

Once the finish has cured and people are walking on it, and the floor goes through seasonal movement (particularly in areas with a great variation in humidity), the places in the finish stressed the most are those where the finish has bridged between the boards. The foot traffic and seasonal movement stress the finish to the point where it releases from the finish below (which wasn’t bonded well enough to start with), and white areas of peeling finish begin to appear. If you use a microscope to look at the finish underneath the peeling coat, you’ll see that there isn’t enough of a scratch pattern to create good adhesion of the next coat of finish.

This problem is much more common in situations when there is more than just a day of dry time between coats of OMU. For example, say you applied everything but the final coat of OMU on a floor in a kitchen, and then the cabinet guys installed the cabinets and the painter was doing touch-ups. It was a couple weeks before you applied the final coat, and now the poly had a chance to cure much harder than it would have over just a day or two. You’ll need to abrade extremely carefully—until the finish is completely dulled down—to ensure good adhesion of the next coat.

Many contractors go far beyond a worn screen to abrade between coats of OMU. One technique is to screen the floor, then come back with a pad and strip system to make sure you hit the lower areas in the floor. Most importantly, after you think you are done abrading between coats, clean the floor and take a careful look to see if there is any visible shine difference remaining (see the sidebar “Lying Low” on the next page). If you see any areas that are shinier than others—board edges are a common problem spot—you may need to complete abrasion by hand until all the finish is dulled down.

Peeling from Solvents

Another problem that can result in white lines between boards occurs with stain or solvent-based sealers: When they haven’t dried well enough before the finish coats, or too much stain or sealer has seeped between boards, solvents trying to evaporate through the topcoats can cause the floor to take on a WLS appearance. On recoats, the problem can be made worse by wax and/or maintenance products between boards that react with the new finish.

WLS from Stretched Finish

Far less common than white lines from peeling is what some manufacturers refer to as “real” WLS. This happens when the finish stretches over a gap that develops when wood flooring boards contract. Just like a black garbage bag stretched too far, stretched areas of finish over the gaps turn white. The white areas are perfectly even, straight lines directly over the gaps.

Because the white lines are a direct result of the finish’s tendency to stretch instead of fracture over a gap, the finish’s flexibility can be a good predictor of potential for WLS. In general, this appears to be more common with waterborne finishes, which are the most flexible, but it also can happen with recently applied OMU. It is uncommon with other finish types and unheard of with finishes such as oil and wax. Because the composition of waterborne finishes varies widely, some may have a greater tendency to develop WLS than others.

Preventing Stretching

Manufacturers stress that acclimating your wood floor, and then ensuring the customer uses good climate control year-round, are the most important steps you can take to minimize WLS from stretched finish. If the floor doesn’t move, the finish won’t stretch and create the lines.

But what does acclimation really mean? The floor should be at a moisture level close to the midpoint of the moisture content (MC) it will have year-round. If you’re installing a walnut strip floor in Ohio in August, the MC of the floor and the subfloor may be within 4 points of each other, but they are probably at their year-round peak, and that floor is likely to shrink substantially come wintertime, especially if the customer doesn’t maintain reasonable humidity levels in the home.

Also, a floor with loose end matches and/or fasteners too far from end joints can cause movement in the floor that stretches the finish.

Additionally, not all finish experts agree, but some believe that using a thicker-than-recommended spread rate may increase the likelihood of WLS.

As always, customer education can go a long way toward minimizing this problem. If you think WLS might be a problem, let the customer know that there is a potential for white lines to appear between boards—at least one contractor actually writes it into his contract. Make sure the customer knows about proper maintenance procedures—wet-mopping the floor, for example, will wreak havoc on MC levels.

If calls about WLS from stretched finish do occur, sometimes waiting for the floor to go through a full cycle of seasons can lead to an acceptable appearance. Another option is to use a razor knife to cut down the finish at the seams, then buff and recoat.

Why Is It Happening Now?

This all doesn’t answer the question of why it seems like suddenly everybody is talking about WLS. The blogosphere commonly cites VOC changes and finish manufacturers changing their products to use different resins. But the resins being used in VOC-compliant wood flooring finish have been used for years, and while finish manufacturers do often change their products, they say WLS also happens on products that basically haven’t changed in decades. VOC-compliant OMU products are thicker than previous versions, with more solids, so it’s possible that longer dry times with those products are making WLS more likely. The reasoning goes that instead of the finish drying and breaking right away, it’s flexible for a longer amount of time while it’s still curing, creating a chance for WLS to occur.

One important factor everyone can agree on is a change in consumers’ preferences. Trends in recent years toward exotic floors, dark floors and plank floors all make WLS more visible. A white line between boards may not even be noticeable on a maple floor, but it will be front and center on a walnut, padauk or dark-stained floor. Plank flooring expands and contracts more than strip, stretching the finish more, and many exotic species are also known for their volatile expansion and contraction on the job site.

Today’s more urgent job sites also may play into WLS. In the olden days, wood floors were installed and sat on a job site for a while before they were sanded and finished. Today, jobs tend to be more rushed than ever, and the wood may still be settling in—in particular, shrinking—while the finish is still flexible and drying.

Finally, customers’ expectations are ever-increasing. As they become more demanding, fewer imperfections are seen as normal, making life more difficult for everyone.

WLS may never totally lose its mystique—and every single case of WLS may not always have an easy explanation—but hopefully by taking the right steps, you can reduce your potential for catching this dreaded disease.


‘WLS’ Caused by Peeling

White line syndrome in which the finish stretches across a gap between boards is much harder to come by than another problem some people refer to as WLS—finish peeling along board edges. WLS from stretched finish is pictured on the opener page of this article. Following are some examples of peeling WLS, which is denoted by white areas that are slightly irregular and actually overlap the board edges.


Lying Low

When abrading between coats of oil-modified polyurethane, make sure all areas of the floor, especially low areas at board edges, have been abraded well enough. Below, sheen differences at some of the board edges show where the abrasive didn’t hit low spots enough. If not abraded further, these areas could be prone to peeling that looks like white line syndrome.

Sources and reviewers for this article included: Johannes Boonstra, Synteko Floor Finishes; Paul Campbell, Glitsa American; Steve Crawford, Dura Seal; David Folkman, Basic Coatings; Bill Jauernig, PoloPlaz; Todd Schutte, Bona US; and Janet Sullivan, Lenmar.

Join the conversation about White Line Syndrome in the HF Forum.

Kim M. Wahlgren is editor of Hardwood Floors.

Wood floor finish    Polyurethane/OMU    Water-based finish    Waterborne finish            Finishing wood flooring    Refinishing wood floors    Sanding wood floors    Wood floor acclimation        Peeling            White line syndrome   


Kim, Great Job. Good explination and pictures to back them up. this goes a long way in helping explain this situation. Maybe you could create a piece of literature we could hand out. Thanks for the help. Reagards Larry
Lary Alfieri  National Sales Manager ACI  6/6/2011 12:40:20 PM

I'm glad to see the NWFA release something about this problem. It seems like it has been swept under the rug for years by people in the finish industry for the most part. I would venture to say that with the current interest in darker floors and low-VOC, that this problem will reach an epidemic soon unless contractors clearly communicate this as a potential problem. I've also commonly seen white lines manifest when people apply too many finish coats to a wood floor.
Scott Avery  Modern Tech Floors  6/6/2011 12:57:38 PM

Well done Kim. A copy of this article should accompany every wood flooring contract signed by a customer. Mike
Mike Hodges  Product Manager  6/6/2011 1:37:25 PM

Kim, great explanations......this is one of those problems I have been involved with recently.....I never really understood which cause I was dealing with.....this makes it much clearer... Thanks
David Harrison  inside sales  6/6/2011 1:55:10 PM

A very well written article to cover the applications for the "white line syndrome". I am very pleased to see you Kim and the NWFA taking the initiative to confirm some of the theories discussed in the industry. Mike is right, this article should be a "must read' for every customer and anyone in the hardwood floor business in general. Thank you.
Runno Allikivi  Arboritec  6/6/2011 3:30:39 PM

That was a very informative explanation Kim. Thanks for taking notice and getting the different perspectives of the manufacturers. This article should help everybody involved with wood flooring. I will certainly pass it along to my finishers. I also completely agree that some manufacturers have more WLS than others.
Rick Menger  Sales  6/6/2011 3:56:17 PM

I think an invesigation into the stains is warrented here. I have noticed this issue for about 10 years now ever since I had to resand a Minwax stained floor that after the Minwax rep showed up at the jobsite I learned of oils being added to the stains to comply with the new VOC laws. Linseed oil is the one I believe was being used by Minwax which made drying time increase by days in some cases. But today even "fast" drying stains are having the same results and Duraseal wont give you a clear answer if you call them. What I have found is that stain sitting in the joints of flooring skins over quickly where exposed to air but remains wet down below. When finish is applied the solvents reactivate the stain causing cross contamination not allowing the finsh to bond properly to the boards.
james  owner  6/6/2011 4:48:42 PM

Did you really think the outcome of the nwfa "independent" investigation would be anything but contractor error? Great to see the finish manufacturers blaming the contractor. Blaming the contractors would help with the millions in potential lawsuits that are coming. Also, the large gap in the story, why is this happening within the last few years. One must ask, what has happened to the industry within the last few years?
Leo  Owner  6/6/2011 6:17:19 PM

Here In Los Angeles with the strict VOC laws in effect we @ restore your floor switched to waterbase finish in 2006 . Using mostly Dura Seal the original all american finish(oil since 1957) we have never had this problem . currently using dura Seal 1000 water based finish you get 500 sqft per gallon & it has the old school look of OMP.we have done over 300,000 sq ft with this product & no complaints NOT ONE . On our Higher end Jobs its a no brainer BONA TRAFFIC will never fail .23 years of personal experience for you in 5 minutes thanks for the interest GOD BLESS AMERICA
John W Rouse III  6/7/2011 4:56:12 AM

It seems to me that in the day of solvent/ non waterbased fillers and putties that ther was less of this phenomenon. My experience with this has also been in previously waxed, oiled,'d, citrus oiled etcetera floors on the recoat/ refinish venue. Have the pretreat processes lessened this problem? Of course that would be hard to tell as you rarely get a callback on a successful job. It is interesting that the photos of the exotic product and the stained white oak show no panelizing affect which makes me wonder about the acclimation question. At least in the Pacific Northwest we see less individual movement and more multi-board involvement. Something keeps the boards from bonding edge-wise, not just lack of acclimation.
Eric C. Jensen  Senior Territory Manager  6/7/2011 11:07:07 AM

This is not a new or more prevelent problem. It's the fact that through the web and the ease of communicating, you're just hearing about it more often. 15 years ago you'd have no idea what Joe Scheleki's Flooring in Timbuktu, but Joe Jr. graduated college and the next generation is here and web savvy. So we all hear about everything good, bad, and stupid. For example, i'm commenting on this because I got the e-mail, read the article and it's too easy to take 5 minutes to type this up.
Poncho  El Conquistador  6/7/2011 11:29:59 AM

Thank you Ms. Wahlgren for bringing this topic to a more public forum. I appreciate having an accredited resource to send customers to when this has occurred on their job.This way they realize that they are not the only one this is happening to. It is also a good reminder for us to make sure we follow proper procedures for our coats. All that being said, I do still believe that there is an aspect of this that still has yet come to light. Conspiracy theories and ease of info sharing aside, we have been in business as a flooring company for 80 years and have not had to talk about this until the past couple years.
Jeremy  Service manager  6/7/2011 1:50:04 PM

The only installation item missing in this detailed article is the installed foundation (base floor). I have specificed "Diamond Floor Finish" (OMU) for more than 15 years here in Southern Califormia. Most of the solid wood flooring was installed direct on a concrete base. One floor is Oak parquet that has been in place 45-years, refinished after a water flood with OMU about 12 years ago. Original installed position, no WLS. The secret may be the application of urethane products with minimum stirring in the can. OMU is not in suspension like paint, it is in effect a complete solution ready for application. I have seen OMU stirred and then applied with more air separation that may cause WLS. The above solid Oak is direct on concrete and Birds Eye Maple Flooring is on shot down 3/4 plywoord (after the flood) in my home with the Diamond Floor Finish (OMU) for more than 12 years - no WLS. ps: I like your acronyms, hope they catch on for inspectors, etc.
B ill Thompson  Construction Manager/Flooring  6/14/2011 10:55:25 AM

FYI I had to start a new WLS thread because know how to add a attached photo to this blog. Anyone wish to read go to:
Roy Reichow  Consultant/Inspector  6/21/2011 6:10:24 PM

I created the name" White Line Syndrome". Started happening for me when the VOC laws changed in So. Cal in 2007. Started using Pallmann Oil as a substitute. The job is hard enough and the "suits" have no skin in the game. They are even saying on their websites that it is a good thing. lol
Greg Warren    7/4/2011 12:18:40 PM

Kahrs Oak Estrella new installation. Several boards out of box WLS Dave Floor Sanding Baine, MN accused me of being too picky!! Great to see documention on the problem.
Sheri Healy  customer  3/14/2012 8:35:42 AM

Thank you for the clear explanation. Now how to resolve it? For "lying low" and "WLS by peeling", I know one way is to refinish the floor. Do you have any easier way?
Frank Yang  Finish Carpenter  5/9/2012 11:34:24 AM

I appreciate you addressing this issue and covering some things that many people might not be familiar with. My question to you was referenced in an earlier comment about using Quick Dry Stains. I have also noticed other contractors stretching there abrasives way too far in terms of screening and coating. I have been a big fan of DuraSeal OMU. I have been fortunate enough to not have run into this issue very often. There are times when I've noticed a slight amount of WLS, but the customer did not see it as a problem or notice it at all. My question about proper abrasion, is what is the recommendation when your attempting to properly abraid a quick dry stain? you reference low spots, but sometimes even with a coat of quick dry stain, as wells a layer of finish on top, using too aggressive an abrasion process will hit high spots and take color out of the floor. so where is the happy medium? Thank you very much.
John Norring  Owner Trinity Hardwood Flooring  2/27/2014 2:06:11 PM

We are a national commercial wood flooring contractor. In a group of stores we are seeing WLS occurring in as little as 30 seconds after we apply a water based floor finish. we have tried 4 different manufactures with the same results. in all cases the stores are street stores and the floors are up to 10 years old. all floors have been maintained by commercial cleaning companies. I would be interested to hear your opinion on cause. Thank you
Brian Prusik   CEO Syracuse commercial floors  5/5/2014 12:36:21 PM

Brian, I would like to answer specifically your question on the commercial floors maintained by commercial cleaning companies. The cleaning companies are paid by the sq ft to maintain floors. To do it fast and with acceptable results to the customer they generally buff the floors with high speed buffers and a product known in the industry as spray buff cleaner and polish. The spray buff compounds are usually low solids acrylic urethanes formulated to melt under the heat of the buffer and polishing pads. Some even have some micro-crystalline wax added for traction. When they melt, they not only polish up on the surface, but some is forced down the cracks between the boards. Applying a water base finish over the compound filled crack will do one of two things, if it is a compatible system it may just have a slight bonding problem over the long term. If it is a very sensitive compound, the water and co solvent will attack the the spray buff turning it white as it tries to dissolve it into the finish. It sounds like this may be your issue. Getting the compound out of the cracks before coating requires a change in preparation procedure and using a machine that cleans in line not rotary, to remove the spray buff before applying the water base finish
Mike Sundell  Consultant  5/8/2014 7:46:16 AM

I sand with new 40 , then with 60 then sharp 80 screen with a new 100 grit , then open windows and doors , then apply hot water with applicator to pop the grain , then let it dry for 25- 40 min longer after it seams dry , then I use carpet on buffer to apply fast drying stain like dura -seal , and after it dries like 40 min I apply zinser universal sealer and bona traffic - one component or an old formula poly right over stain , or synteko clasic 35 I stay away from the 2 component or low voc - cheap quality floor governmeent conpliant bs , they give me headache more then regular poly and during the whole process when I walk on the job I install air-king powerful window ( whole house ) fan I usually have no white lines and exotics like walnut - they contain oils , so just use synteko classic 35 and wear proper mask and open windows , turn of any pilot lights very highly flammable and combustible !
Polski floor  owner  5/16/2014 9:57:38 PM

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