Sealer or Self-Seal? 8/14/2012 10:00:00 AM 
Should you use a sealer or the topcoat self-sealing? It depends on a few things: the manufacturer’s recommendation to use a separate sealer, self-seal or both; the type of coating; and the physical characteristics such as dry time.
The most important issue is how the manufacturer recommends the products are used. Manufacturers test their products in recommended finishing systems; the number of tests interior wood finishes may need to pass is significant. Varying from these recommendations becomes a trial and error experiment for the finisher and will probably negate any liability of the manufacturer if your results don’t meet your expectations. Manufacturers tend to recommend sealers similar in chemistry and/or durability. This prevents the foundation (sealer) from being weaker than the topcoat.
A simple lacquer finish will usually have a separate sealer. Lacquers don’t cross-link to produce as hard a film like catalyzed coatings do, so sanding many lacquer topcoats is difficult. Usually the lacquer sealer will need a significant amount of sanding aid, usually a zinc stearate. Zinc stearates provide significant “powdering” when sanded and the powder provides lubricity to make it easier to sand and less “gumming up” of the sand paper. Note: Stearated sealers should not be used under catalyzed coatings as “blooming” can occur. Blooming is when the acid catalyst in the topcoat reacts with the zinc stearate and creates a slight layer on the surface of the film resembling a haze. This haze can keep coming back after cleaning. Some sand paper is coated with zinc stearated (usually light gray) for easier sanding.
Catalyzed coatings, including pre- & post-catalyzed lacquers and catalyzed varnish, may have a separate sealer. If the catalyzed topcoat sands well enough and dries and cures in the time needed, it will probably work well for self-sealing. However, when the topcoat does not meet the criteria for the sealer, a separate sealer is needed. Many times a separate catalyzed sealer is needed on production lines to meet the dry times before sanding. Catalyzed sealers should not contain zinc stearates to avoid blooming. They use different sanding aids that provide the powdering and lubricity needed.
Choosing whether to use a separate sealer depends on the manufacturer’s recommendations, the types of coatings and whether the topcoat will meet the working characteristics, such as dry time to sand, you need.
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Re:
Created by Maureen Alley in 8/31/2012 9:13:06 AM
Daniel, thank you for the comments. Blogs are meant to be short by nature and will tend to hit the high points of the subject matter. One goal of the blogs I write is to provide some information that will lead to many questions that should be asked of your finish supplier. It is the communication between finisher and supplier that needs to be in depth, relating many specifics of the particular finishes and processes to gain a deeper understanding.
There are many issues beyond the information provided in any blog that affect sealers and topcoats which are best explored in depth with your finish supplier. Another way to gain more information is from finishing books that relate to your type of finishing. I recommend “Understanding Wood Finishing” by Bob Flexner for those who are serious hobbyists to small shops, and even mid to large finishers can gain useful information from it.
The specific coatings that I cover will be those that are widely used in industry on kitchen & bath cabinets and furniture for offices, hotels / motels, and healthcare facilities. Shellac-based products are not durable enough for these applications. Definitely contact your supplier of shellac products to learn more about them.
Which finishes should be used together? Use the finishes recommended as a system by the manufacturer. As a greater variety of finishes are available it is tempting to just mix and match as the finisher sees fit. However, in evaluating many finish failures a common theme is using the wrong finishes together (finishes not recommended together) or using them in a way they were not intended (such as using a wiping stain as a toner coat to darken the color). This subject is far too complex to cover in detail in any blog and should be taken up with the finish suppliers.
-David Jackson
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Sealers? Or Self-Sealing Topcoat?
Created by Daniel Shafner in 8/14/2012 5:12:45 PM
I expected this article to have a little more depth or provide a little better insight into sealers. What I learned from the article was that for production purposes and for purposes of being able to grind the foundation coat to a smooth film, then a stearated sealer is a benefit. OK.
Are those all the issues with sealers? Or perhaps I misunderstood the article and the writer only wished to focus on coatings which can either be used with sealers or can also self-seal.
In general, most film-forming coatings can be used as their own sealers. That's in general. An article in a trade publication would want to enlighten us a tad above just what's already know "in general". No?
What about shellac-based sealers? They have their place and there are many reasons to use a shellac for a first coat. What about glu size? That's a sealer, of sorts. What about polyester sealers under lacquer topcoats? Under Polyurthane topcoats?
Here's one thing I saw missing, the issue of adhesion to the substrate, some sealers are designed to adhere because perhaps the topcoat molecule is too big and to complex for it to "find a home" in adhering to the substrate. That's an issue I saw missing and the beauty of shellac is it's versatility in adhering to all kinds of substrates, plus is ability to act like a barrier coat. It makes a great tie coat.
Waterborne coatings are of particular interest and concern since they are and they continue to be the bastard child of the finishing trade, as well they should. Some waterborne coatings suggest to use their sealers and some suggest that they can be used as self-sealing systems.
In short, it's more than just dry times and sandability, I think. It's also adhesion, sizing, tie-coat possibilities and barrier coat possibilities. The finisher must decide, though, will using the sealer aid him in his finishing? Will it speed up the process? Will the sealer detract from the durability, performance or ho
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