Much of the available information on finish and number of coats is contradictory and often misleading. And it easy to see why. What makes it hard is that you can’t choose finish using your senses. Most things in life such as furniture, car, phone etc can be touched and seen. A can of finish is chemistry. You can’t see it, you can’t touch it, you can’t compare it. So, a manufacturer can sell you a can of 100% polyurethane finish when it really isn’t. You don’t know.
To make things more complicated things like room temperature and humidity during application, sanding methodology, finish application method, coat thickness can all affect the product performance. The lack of visual confirmation coupled with many of these uncertainties enables contradicting experiences and myths to spread.
Truth is there isn’t a simpler answer for a number of coats that should be applied. But, looking at the basic facts and underlying science can make that decision easier. But first let’s establish some rules to ensure that finish is being applied correctly.
Finish Application Factors
- Sanding Grit. Floors need to be sanded to 100 girt or higher. Low sanding grits leave wood grain open allowing most of the finish be directly absorbed and leave less finish on top for protection.
- Sealer. A proper sealer was used before finish application.
- Ambient Air Temperature and Humidity. Ambient condition can slow or prevent finish from curing properly thus affecting it’s future performance.
- Cure Times. Applying coats to quickly may speed up the project and lower the cost. But, if coats are applied too quick, finish won’t be able to cure correctly and under-perform in the future.
- Coverage Rate. Finish manufacturers specify a spread rate for their products. For example, most Loba brand products need to by applied at a rate of 500-600 sqft per gal. Lack of experience, use of wrong applicators or sometimes simple cost cutting leads some companies to apply finish much thinner in 700 – 900 sqft per gal range. This is extremely common and leads to a premature finish wear and poor performance overall.
- Product Quality. There are hundreds of finishes available for sale. Prices can very from $20 to $140 per gallon. Some brands will offer extremely high quality, while other can be substantially less durable. Big box stores tend to carry products that are easier to apply and cost less.
Number of Coats
Most finish makers specify that two coats is a minimum number necessary to achieve a durable film. This means finish will have an even sheen, offer adequate protection against regular traffic, spills etc for a certain period of time. It is telling that the period of time for which finish will last isn’t specified.
The reason is that it depends on how many people live in the house, level of traffic, number of pets, maintenance etc. But, by establishing these factors it should be easier to determine what finish, and the number of coats needed to provide the best value for a dollar. For example retired couple with no kids or pets living with them can reasonably expect 2 coats of regular finish last 10 years or beyond if floors are maintained correctly. At the same time I’ve seen floors in rental homes that have 3 coats of high traffic finish destroyed within a couple of years. Average family can expect somewhere between 4-7 years before wear areas begin to appear.
It helps to look at finish film thickness for answers. Let’s do some simple math.
One gallon equals 231 cubic inches. If we follow Loba brand guidelines and apply finish with a rate of 550 square feet (79,200 square inches) per gallon, would expect a wet layer thickness of 0,00292 inches or 2.92 mil (mill = 1/1000 of inch). Solid content of finish is 37%, so a thickness of fully cured coat of finish is 2.92 mil x 37% = 1.08 mil. However, part of the first coat will be lost to buffing and grain raise. Let’s estimate that loss to be be 0.3 mil.
So, if floor had two coats of finish applied to it, a film layer protection thickness is going to be 1.08 mil x 2 – 0.3 mil = 1.86 mil. And, for three coats it is going to be 2.94 mil. This means you would expect floors with 3 coats last 64% longer when compared to 2 coats before needling re-coating or refinishing. This can translate to little over a year when using our poorly maintained rental home example from above, 6.5 years for a home with a relatively low traffic, or around 3 years for average household.
What About High Traffic 2-component Finish?
High traffic finishes such as Bona Traffic or Loba Supra are two-component finishes. Which means that a cross-linker (hardener) is added to the polyurethane polymer. A cross-linker chemically creates more bonds between polyurethane molecules called monomers. There more bonds there are stronger they can hold together, the stronger the finish is.
We don’t have any test data to show how this translates in finish durability in terms of years. But, if make educated guess of added 20% durability. That will mean 1 year of additional finish life for average household with 2 coats of finish, and 1.5 years for three coats.
High traffic finish has additional benefits such as better scratch resistance, and better resistance to household cleaners which in a way can be more important than better abrasion resistance discussed above.
You can learn more about crosslinking in wood coatings on PCI website.
Choosing number and coat and type of finish is very personal and will change depending on span of time you need finis to last, amount of traffic, and maintenance. For example if you don’t expect high traffic and planning to sell the house within 5 years or so, two coats of regular traffic should do just fine. A new coat of finish to refresh it might be all that is needed to make the floor look newish again. If you have a big family, dogs and want the finish to last as long as possible, then 3+ coats of high traffic is the best bet.
It is worth mentioning that dents, gouges, scratches etc can happen if enough pressure is applied no matter the finish or number of coats. If floors are oak, cherry or most other domestic wood species, a heavy dog sliding is sure to leave indentation scratches. Harder woods such as Brazilian cherry or Brazilian teak are much better at preventing such type of damage.