By Ben Totta
White lines. If you are a contractor who has ever had that issue on a floor, or ever received a call from a customer describing this issue on a floor, that term might make your skin crawl a little. Stemming from the reality that once this issue is apparent on a floor, often the only cure is breaking out the sanders and refinishing the floor.
The two major causes of white lines are from finish film stretching, and from finish film lifting. According to the NWFA’s Problems, Causes, and Cures publication, white lines are caused as a result of finish or sealer that stretches, rather than breaks, along board edges or seams, with seasonal humidity and temperature fluctuations. I won’t dive into different finish types, but particularly finishes such as 2K isocyanate hardened water based finishes, which remain quite flexible after curing, can be prone to stretching across a seam. When the film stretches, like when you bend a credit card back and forth, it turns white. This does not necessarily affect the durability or performance of the finish, but it is unsightly, particularly on a dark floor and may need to be addressed.
Finish lifting also gives the appearance of white lines. Basically, it is the finish lifting or separating from the wood floor beneath. This adhesion breakdown is most commonly caused by trapping solvent, which has not fully off-gassed, underneath the coat. A loose floor can also have a lot of movement between adjacent boards, which over time can lead to this lifting along the joint between two boards. Lifting finish has the potential to affect durability. If bad enough, that lifted film can start to peel from the floor, which would require resanding to remedy.
So how do we try to prevent white lines?
1. Create and maintain a stable floor.
- Proper conditioning of the wood floor in relation to its environment and proper installation methods need to be used.
- Educate the end user on the importance of conditioning the environment to limit the relative humidity swing throughout the seasons.
2. Allow ample dry time for all steps of the finishing process.
- This includes water popping. Any water which is not allowed to fully evaporate will be trapped and increase dry time for everything coming after it.
- Stains (or anything containing solvent) may dry relatively quickly on the surface of a board, but when it flows between flooring boards can take a very long time to dry and can easily lead to solvent lift. Additionally, heavily pigmented stains will generally require even more extended dry times.
- Follow manufacturer recommendations for applying any finish, noting dry times, and how much time should be required between coats.
3. Avoid an excessive build of finish.
- The thicker the film build, the more prone it is to stretching over a seam with normal wood movement and creating a white line instead of snapping. More coats doesn’t always equal a better finish. Follow manufacturer recommendations.
4. Choose flooring and finish combinations that are a good fit.
- Some floors are going to have more movement. Consider using a finish that is less prone to white lines in these situations if possible.
- Don’t hesitate to discuss the potential for white lines with a customer, educating them beforehand is much better than trying to explain the issue after. Also don’t be afraid to turn down a job if someone insists on it being done in a way the you think will be prone to white lining.
According to the NWFA’s Problems, Causes, and Cures publication, it’s important to address any seasonal fluctuations or movement between boards prior to addressing the white lines. In situations where seasonal fluctuations or movement within the flooring system are unavoidable, use a less-elastic finish to minimize the effects.
To address a white-lined floor you need to determine if it’s due to stretching or lifting of the finish. If it is lifting, quite likely the only fix is resanding. If it is simply stretching, you may be able to touch it up by cutting the film stretched over the seam, and touching up with a stain marker. This will hide the white line fairly well, but is quite tedious and time consuming if the area is very large.
There’s no such thing as a perfect finish product or type of finish. They all have their pros and cons and place. Educate yourself on when they will perform well, and confidently educate your customers.
NWFA. (2018). White Lines. Problems, Causes, and Cures, p. 52-53.