Water-Based Lacquer Versus Water-Based Urethane

Properties vary within product categories, as well as across them. March 18, 2005

Question
I am curious to know if there are great advantages to either water-based lacquer vs. water-based Urethane. I've been using urethane and am wondering if there is a reason one might switch to using lacquer.

Forum Responses
(Finishing Forum)
From Paul Snyder, forum technical advisor:
It depends on the properties you need from the finish. If it acts like solvent lacquer, the water-base version should have 100% burn-in between coats, be optically clear, but not the most durable finish. It’s good in situations where you want to rub out the finish without witness lines between coats and/or clarity is an issue. If rubbing out the finish isn't important and you want the added scratch, chemical, and heat resistance, then the urethane is the better choice.


From the original questioner:
I've noticed the term "burn in" used frequently. What exactly is this? Is this typical with lacquers only?

From contributor D:
Each water-based product is its own animal. Rather than be concerned with what resin is inside the globule of water-based finish you only need to know certain properties of that particular finish.

Calling a water-based a lacquer or polyurethane is more a marketing ply than it is an aid to classifying the finish. These are coalescing finishes and each product has its own behavior characteristics. There are pluses and minuses to every one of them depending on what you need the coating to do for you. Discuss your uses with the tech supports for each of the manufacturers and let them guide you with your product selection based on what your priorities are and which coatings will meet those needs.

The ability of a coating to "burn in" or "bite" into the coating underneath it is a means of that coating gripping itself to the undercoating in a chemical way, not by mechanical means.

When a coating burns in there is enough solvent action taking place between that coating and what's underneath that the coating softens and/or melts to whatever degree the coating underneath and chemically meshes with that coating.

A mechanical grip is more like the molecules of coating getting "stuck" in the scuffings and sand scratches of the undercoating. The gripping action is mechanical in that sense. Scuffing between coats helps to insure inter-coat adhesion even with materials which you assume might grip chemically (burning themselves in). Those are the two ways to deal with inter-coat adhesion, burning in and mechanical gripping.


From Paul Snyder, forum technical advisor:
Some finishes form a distinct layer with each coat. If you sand these finishes back to level them out you will likely cut through one or more of the layers/coats. When you do, you will see a faint outline where you sanded through each layer. These are called witness lines.

Other finishes fuse together to form one continuous film as you apply each coat. These finishes have 100% burn-in. If you sand these finishes back, you will not see witness lines. Shellac, lacquer, and some water-based finishes burn-in between coats.


From the original questioner:
I am now wondering about the finish I have been using. It seems that when you set a paper plate with relatively hot food on it, it makes the finish blush. Could this be because it is not a 100% burn in type of product? I was told by the manufacture that there is moisture caught between the sealer coat and the topcoat. I know I have a dining table that often gets this scenario to the test and has never shown signs of this blushing..

From Paul Snyder, forum technical advisor:
That sounds like a heat sensitivity problem; not really related to burn-in. That is one weakness that water-base finishes have; they're not as heat resistant as finishes that cross-link (varnishes and catalyzed finishes). Some are better than others though, and it sounds like the one you're using has a fairly low temperature rating. Moving into a urethane or possibly another brand with a higher temperature rating will help resolve the problem.

What happens is the heat causes moisture to get trapped in the finish and makes it blush (turn milky looking). A lot of times the blushing will take care of itself as the moisture escapes from the finish. Nitrocellulose lacquer and shellac also share this problem. If you need a really durable finish for tabletops and the like, conversion varnish is a good choice.

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