Sealcoat Before a Waterborne Finish

Water-based finishes tend to raise wood grain. Here, finishers discuss options for a first sealer coat. October 25, 2006

I have been using WB Target on some commercial projects and found that seal coating on the first coat eliminates grain raising issues, etc. Does putting seal coat down as a first step effect the integrity of final finishes? There have been several discussions here about the ineffectiveness of putting a harder finish over a softer finish. I've had horrible past issues with Zinnser's durability, but like it as a primer. I guess I'm worried that if I use their seal coat, I may also have durability issues on final coats.

Forum Responses
(Finishing Forum)
From contributor M:
The intention of seal coat, as you said, is to prevent grain raising; it is not to be used to build it up into a heavy, thick sealer. Because shellac is such a terrific sealer, very little is needed - a wash coat or two in most cases - and it will not affect the integrity of the WB coating, providing you apply the WB coating as intended by the manufacturer.

From contributor J:
Why not use Target's sealer too? If you're still not sure, I would ask at their forum or give them a call. I find that Target is incredibly helpful with info on their products.

From contributor M:
I think that Target has its own water base shellac.

From contributor R:
Isn't waterborne shellac a misnomer? How can you dissolve shellac in water?

From contributor M:
This is a Target product: Oxford UltraSeal-WB. A water-based dewaxed shellac finish designed to function as a sealer and barrier coating for use on wood construction and restoration applications.

From contributor M:
Simply put... Water will go into alcohol, and alcohol will go into shellac. In this particular process, specific alcohols are used for this product. Basically, it's the same process that creates the water base coatings.

From contributor B:
Zinsser SealCoat is an alcohol-cut dewaxed shellac that can be cleaned up with water if you first neutralize it with ammonia. A smelly procedure, and you are still dealing with a flammable product (SealCoat) along with the strong odor/danger of ammonia. SealCoat works very well under WB finishes and will not harm the final hardness of the WB topcoat if you apply at least 2 coats to get a good dry film build.

The Oxford UltraSeal-WB is dewaxed shellac resin dispersed in water through some sort of pH adjustment process that Target Coatings developed. This process makes the UltraSeal 100% water soluble and 100% non-flammable. This product makes for a great undercoat for all WB's - it seems to melt right into the topcoats and helps the topcoats level better. I don't understand the Target chemistry, but I do know that this product and others that they make work very well indeed.

Whether the dewaxed shellac is dissolved in alcohol or water, the final result when either solvent evaporates is a dewaxed resin base coat. The end results between the SealCoat and the UltraSeal-WB are identical, with a slightly lighter color produced by the UltraSeal-WB, and a bit of grain rise from the first coat...

From the original questioner:
I agree that Target sealer is awesome; however, my issue is that slight grain raise creates more work in a closed grain finish. For best penetration, 25% water (distilled) is added to Target ultra-seal. That extra water is going to raise the grain. Zinsser's durability issue has been resolved, but grain raising has not. I've even tried pre-grain raising before sanding, with no luck. Maybe a very, very light first coat of wb sealer with resolve the issue, but with 25% water added, I doubt it. Thanks everyone for your responses... I will mess around with both approaches again. Learning curves stink!

From contributor R:
Pre-wet your wood with water before your final sand and your grain raising issues will be very minimal, as well as getting rid of dents and scratches.

From contributor M:
Contributor R, that's the very reason that they use the shellac; they don't want to wet all the woods that make up suites, and then have to wait for them to dry, and then sand all the woods. Shellac can save the finisher a lot of work and time, be it alcohol or WB shellac.

From contributor R:
I'm not talking about submerging the wood in a vat of water, just pre-wetting. I have never had to wait more than 10 minutes for it to dry and the time saved by getting rid of dents, scratches, grain raising (we use Lockwood water based dye here) and increased lacquer adhesion is worth the effort for me. Personally, I don't use shellac under lacquer unless I absolutely have to. I like sticking to the same manufacturer all the way through. I find that a thin coat of vinyl sealer will work just as well in 99% of the situations where a sealer is required. I have done adhesion tests with our lacquer over shellac and was not impressed. I know a very thin coat allows the lacquer to still bite into the wood, but I prefer a vinyl so I also get a chemical adhesion as well.

From contributor M:
Whatever works for you, is best for you!

From the original questioner:
I agree with contributor M. 1) Whatever works best for you is great! 2) I have tried a barely damp cloth just before final sand and it does help a lot, but I still have grain raise that has to be removed. The only thing I wonder is, when I do that damp cloth before final sand, does it trap dust into that open grain it creates, and then lead to fish eyes?

Possible solution: Does anyone know if I have some raised grain left? Shoot my sealer, no sanding, wait, shoot another coat of sealer. Will there be enough build from the two coats of sealer that when I do sand, it will be smooth and grain free? Basically, will two coats of wb sealer equal a pore filler? I know one coat of sealer is not enough even with pre-damping before final coat.

From contributor M:

This would depend on the wood you use and the solids content in the coating you use. (In closed grained woods, in most cases, yes.) Just remember that sealers are not meant to be paste wood fillers. They make a poor base when too many sealer coats are used.

From contributor M:
I spray the water on with a cup gun and just wipe off the excess with a rag if needed. If you blow off before spraying, there is no sawdust trapped and even if there was, when the water dries, it would just blow out anyway.

From the original questioner:
Thanks. I will try the water trick again. Not a big fan of Zinsser.

From contributor J:
If the wood you are using does have closed grain, you could probably get away without pre-wetting and sanding again. I do a lot of work in hard maple and when finishing, I'll sand to 150, spray two coats of sealer, scuff with a 220 grit sponge, then shoot my topcoats. Usually I'll only go through the trouble of raising the grain when it's more open, like mahogany or walnut.

From contributor T:
Q: What is Hat Makers Varnish?
A: Water based shellac.
Q: Why isn't it used on wood much?
A: Cuz it raises the grain, which alcohol based shellac does not do.
Q: Why am I advised not to use an alkaline soap to clean my shellacked furniture?
A: Cuz alkaline solutions will dissolve the shellac.
Q: Can I make WB shellac?
A: Yes. Dissolve shellac flakes in an ammonia or a 20 Mule Team Borax solution. Either of these solutions can also be used to clean shellac brushes, if you clean your shellac brushes, regardless of whether the shellac is WB or AB. No alcohol is required. Note that the shellac is dissolved, not suspended, as in most other WB finishes, and therefore WB shellac remains an evaporative finish.
Q: Is dewaxed shellac hard or soft?
A: Wax is a natural plasticizer in shellac. Removing it tends to make the shellac harder and more brittle. I don't know if Zinsser compensates for this or not, but if you make your own dewaxed shellac, you can compensate for it by adding a few drops of caster oil (up to 3%).

I would recommend that you stay with seal coat which is one of Zinsser's better products. It's a great barrier coat for lots of things, including WB finishes that tend to raise the grain, and it comes from a can (no messing with dissolving and dewaxing your own). You shouldn't see any degradation of your final finish either. I'm not sure of what the Ph is for WB finishes but if it's basic, the finish will probably melt into the shellac a little bit - a good thing.

From the original questioner:
Just to let you all know how it finished: I was doing a low budget interior 3' fence/knee wall. I was using poplar for solids and birch plywood for wrapping posts. I ended up tinting the Zinnser sealcoat slightly and priming the poplar to match the birch... Came out almost perfect. The birch I just primed with Zinnser and no tint. I finished with the Target and it looks great. Thanks for the help.