We may receive a commission when you use our affiliate links. However, this does not affect our recommendations.
An explanation of this most confusing of the finishing treatments.
IIn the late 1980s, I hosted a local radio show that dealt with finishing and restoration. I remember a caller explaining that he had applied four coats of tung oil; asked what he should use to seal the wood!
OK, so he was probably using a fake cleaning varnish labeled “tung oil”, as I’ve explained many times, but four coats of any finish will seal the wood pretty well. A single coat, in fact, is sufficient unless the finish has been excessively diluted.
Clearly, this guy misunderstood the term “sealing”. But he is not alone; there is probably no deadline that is more misunderstood.
A bit of history
In the nineteenth and early twentieth centuries there was no talk of sealants and, as far as I know, there were no products marketed as sealants. The likely explanation is that almost everyone, including furniture makers and painters, has used shellac, which is fairly easy to sand.
But in the 1920s, nitrocellulose lacquer was introduced which replaced shellac in furniture factories. Unlike shellac, lacquer is not easy to sand down because it tarnishes the sandpaper, causing small lumps called “calluses”. Since the first coat must always be sanded to remove the roughness, the manufacturers have created an easy-to-sand finish, which they have called “smoothing sealer”. The name is quite logical when you remember its purpose.
Paint has a similar gumming problem to sandpaper, so as the market for this finish grew, sanding sealants were also developed.
So what is it that makes the sanding sealant different from the finish itself: lacquer or paint? It is the addition of zinc stearate to the finish. Zinc stearate is a type of soap. Think about what would happen to the dried finish film if you added hand soap features to the finish.
The film would be smoother, so it would be less likely to clog the sandpaper. This is good. But then consider the negatives.
■ The film would be less resistant to moisture and liquids.
■ The film would also be softer, so it would scratch more easily.
■ The film would break more easily due to the reduced plasticity.
■ Any finish applied to the sanding sealant may not adhere equally.
These qualities are easy to imagine once you know that zinc stearate is soap. Sanding sealant is therefore a compromise between easier sanding and these four negative qualities. The conclusion is obvious: you endure the toughest sanding of lacquer and paint unless your project is large, in which case the time and effort saved, along with the savings on sandpaper, are worth the trade-off.
So factories and cabinets typically use paint sanding sealer, but it is rarely necessary to use a paint sanding sealer or paint sanding sealer unless the project is large. You can make sanding a little easier by sanding with stearated sandpaper.
If you run out for a decade or more, you’ve no doubt noticed that polyurethane paint has largely replaced traditional alkyd paint in stores. Polyurethane cures harder than alkyd paint and does not bond well to the soapy abrasive sealant. If the finish takes a hard hit, the polyurethane can separate.
As the polyurethane market has grown, manufacturers have become more aware of this problem and have begun to warn against applying it to a smoothing sealer. But the carpenters had been conditioned to the need for a special sealant, so shellac was increasingly promoted. After a while, it became apparent that polyurethane did not bond well with shellac due to the wax it naturally contains.
So Zinsser (Bull’s Eye), the only remaining supplier of liquid shellac (as opposed to flakes), created a dewaxed version called SealCoat, which they marketed as a sealant for polyurethane. It works quite well, but consider how absurd it is.
Polyurethane is very easy to sand, which means it dusts and doesn’t clog the sandpaper. So no special sealant is needed. A first coat of shellac, in fact, can only have the effect of reducing the protection and durability of the total film. Why should you do this?
Starting in the early 1990s, water-based finishes became widely available. Water-based finishes also dry well and sand easily. But the market has been conditioned by the need for a separate sealant. Then the manufacturers supplied a water-based sanding sealant. Whether this was due to their ignorance of the “seal” or because they simply wanted to sell you another product, or both, you can decide.
However, they came up with quite ingenious reasoning for the sanding sealant. They claimed that grain farming was reduced. The water-based finish is slightly alkaline and the alkalis increase the leavening of the grain. So the smoothing sealants have been made, or at least claimed to be made, more acidic. This shortened the duration, but it was presumably worth it to reduce the grain lift.
I have never been able to tell any difference in grain farming. I have to assume that others have had the same experience (or lack of it), because manufacturers seem to have given up on this explanation altogether. Some, targeting the woodworking market, still offer a water-based sanding sealant, but you don’t find it often in paint shops or home centers. There is simply no need.
Diluting a finish (varnish or lacquer) also facilitates sanding because the thinner film hardens faster. So sometimes you see the instructions for diluting the first half of the coating with the appropriate thinner.
But the explanation is rarely easier to sand. Instead, it is better to tie. The wooden boat community, in particular, treasures this explanation.
While deeper penetration leading to better bonding may seem intuitive, I’ve never seen any evidence for it. The reason the peel runs out is not because of a lack of penetration. All finishes bond well. It’s because moisture gets underneath the finish and the best way to keep it out is with a thicker texture. Diluting the first coat reduces overall buildup unless additional coats are applied.
In summary, it is best not to use sanding sealer or shellac for your first coat (sealant) unless all of your coats are shellac or your project is large and you are finishing with lacquer or alkyd paint. And there is no reason to dilute the first coat.
Here are some supplies and tools that we believe are essential in our daily shop work. We may receive commission from sales sent by our links; however, we have carefully selected these products for their usefulness and quality.