There are several factors that influence the quality and performance of water based polyurethane and water based lacquer film finishes, including:
Environment (temperature & relative humidity)…
And of course, knowledge and technique.
An important part of knowledge and technique is knowing and applying the optimal thickness of finish to each coat, and the total thickness of all coats combined. .
In Part 1 of this article, I will explain what wet film thickness is, why it is important to achieving a good performing finish, and how to measure it. In Part 2, I’ll cover dry film thickness.
What Exactly is Wet Film Thickness?
Simply stated, it’s the thickness of a coating, pigmented or clear, measured right after application and prior to any drying taking place.
Why is Film Thickness Important?
Wood finish manufacturers engineer their products to meet specific objectives, and because each product has its own variety and percentages of materials in its mixture (as well as how they are brought together) each behaves and performs in a unique way.
Consequently, each product will have its own application guidance, which includes the specified thickness for each coat. They will also specify the total thickness to meet their performance targets for that finish. Depending on the type of resin and solvent chemistry used in the formula, the manufacturer may specify a wet film thickness ranging from a fine 1mil tack coat upwards to a heavy 6mils.
Keep in mind, too, finishes flow out best at a specific thickness.
A coat that is too thin won’t level out properly and will have a rough texture to it. Conversely, if it’s too thick, it won’t dry correctly, will entrap air bubbles, and it may run.
And in the case of certain water-based wood coatings, it may exhibit a blueish or whiteish haze.
For these reasons, it is important that the optimal level of thickness is applied for each coat, over the entire area.
What is the Optimal Wet Film Thickness?
You can usually find the finish manufacturer’s recommended thickness for each product on their website’s tech data sheet.
As an example, looking at the tech data sheet for Target Coatings EM6000 production lacquer, if you scroll down to “Application on Unfinished/New Wood,” you’ll see it recommends applying “2 – 4 mils per wet coat”. (A mil is 1/1000 of an inch, or .001”)
Measuring Wet Film Thickness
To measure wet film thickness, you can use a wet film gage, or “comb” as it’s often referred to.
Find the edge of the gage with the range you’re measuring (1 – 6 mils for most wood finishes).
Right after spraying a coat, place that edge of your gage perpendicular to and touching the substrate. This can be done either on a test panel, or in an inconspicuous area of your piece.
Hold it in position and wait a few seconds until the teeth are wet.
Then, remove the gage from the film and look at the teeth. The wet film thickness lies between the largest value that is “coated” (wet) and the smallest value that is “uncoated” (dry).
Here’s a tip when measuring clear finishes: it can be difficult to see any difference between wet and dry teeth, so, after touching the gauge to your work or test panel, scrape the gauge an inch or two along the surface and see which notches make a trough in the finish. You can remove the gage marks by lightly spraying a second, thin wet coat over the gage test marks – melting the coats together.
To maintain its accuracy, be sure to clean off your gauge with the appropriate solvent (use tap water for water based products) as soon as you are finished taking a reading.
Bellow is an excellent video explaining wet film thickness gauges and how to use them…
I hope the information above was helpful.
In Part 2, I’ll cover dry film thickness and how to accurately calculate it.
In the meantime, do you have any questions on wet film thickness, how to measure it, manufacturer guidelines, etc?
Please share your thoughts or read what others are saying in the comments section below.
Marty Schlosser has nearly 50 years experience as a furniture designer, maker, and finisher, including a career in production-level spray finishing. Often called, “the industry’s go to expert,” Marty is a past-president of the Ottawa Woodworkers Association (and a proud recipient of their Danny Proulx Memorial Award), and the founding member of Kingston Wood Artisans, a local woodworking club in Kingston, Ontario, Canada where he and his wife reside.