Sanding sealer....when and how to use it?

[Quesne Ouaques]

I have used sanding sealer in the past to get a smooth, sealed surface before varnishing (I usually use rub-on polyurethane). I have had mixed results, however, with respect to the final quality of the finish and getting a natural look out of the wood grain.

To educate myself a little bit on when and why to use it, I was hoping to get some feedback from the folks at the Creek. Any help you can give me in getting answers to these questions would be appreciated!

1. What is sanding sealer for, and why isn't it a more common topic of discussion on the woodworking forums?

2. How is sanding sealer best used and under what circumstances do you get the best results?

3. What finishes are most compatible (i.e. give the best results) when following sanding sealer?

[glenn bradley]

I think if you do a Google search here on 'Seal Coat' you will get a lot of hits. I use Zinsser's Seal Coat which is their name for dewaxed shellac. I hit it 1:1 with DNA when using it as a sealer or to pre-treat cherry, maple or red oak to avoid blotchiness with oil based stains.

[Jim Becker]

"Sanding Sealer", in the marketing sense, is a product that is best suited to filling the finish vendor's pockets with extra profit. With few exceptions, it's just plain not necessary and in some cases, it can actually negatively impact the "durability" of the finish. (it may be a softer product than the final top coats and provide a weak base) So you get to spend more money on a product you don't need that might give you a poor finish over the long haul... A very high percentage of today's finishes are "self sealing" and don't need any extra help in that regard.

One of the exceptions would be certain types of sealers used in the lacquer world that are formulated to work with the subsequent lacquer products. They are formulated specifically to work with that same vendor's product, but contain higher solids for better filling, etc.

That said, for general sealing between different kind of materials, de-waxed shellac is almost a "perfect" product compatibility wise. And example would be where you use a water soluble dye to color the wood. If you want to top coat that, especially with a water borne product, sealing the dye with the de-waxed shellac will keep you from re-dissolving the dye and possibly causing noticeable degrading of your work's appearance. Another example would be where you want to put polyurethane over certain other materials and you actually want it to adhere to the workpiece. (Poly hates to stick to itself, even, sometimes....)

[Steve Schoene]

What Jim said. The best sealer is always the top coat except in those special production shop situations he mentioned.

But if you have different materials to keep separated dewaxed shellac works well. Other products labeled Sanding Sealer often contain stearates--a form of chemical soap to make them sand easily, but which weaken the over all finish, and make some finishes adhere poorly.

[Howard Acheson]

Let me add that true sanding sealers must be matched to the top coat. Lacquer based sanding sealers contain stearates which inhibit adhesion of oil and waterbourne finishes. Lacquer based sanding sealer should only be used under lacquer finishes. There are some oil based sanding sealers but they dry rather slowly. Finally, certain high tech finishes require a sanding sealer but you must use the one supplied by the manufacturer.

While Zinsser labels their SealCoat as a "sanding sealer" it really isn't. Sanding sealers are used in production to build a finish rapidly. To do this they have lots of soft solids that fill surface depressions and build a film. Lacquer has very little in the way of solids so it does not fill defects unless lots of coats are applied. Shellac like SealCoat also has very little in the way of solids. Therefore, it takes lots of coats to fill surface in woods like oak, mahogany etc. So in that sense, it's not a very useful product as a sanding sealer.

[charles anderson]

I use vinyl sandsealer in my woodworking business. The parts I make are made of ash, and after staining with a lacquer based wiping stain, I spray with vinyl sandsealer. I sand with 220 grit paper, and apply a final coat of conversion varnish. This gives me a high gloss, durable finish. I've been using this system for many years, and the sandsealer gives me a mirror like finish. Good stuff.

[Jim Becker]

Charles, that's an example of one of the "matched systems" where the sanding sealer really is a good part of the picture. But it's one of the few...