This is a bit off topic, but I just coated two wooden chairs made of pine with shellack to seal the knots. Now I am wondering what to put on for the final finish. I would like it to be clear so as to show off the wood grain. The Shellack bottle said not to use polyurethane over shellack. I plan to keep the chairs outside on the pattio. Any help would be appreciated.
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What is a good cover coat for shellack?
- Thread starter Tom Lewis
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It is not a good idea to put poly over shellac unless it has been dewaxed. You can usually start with Super Blonde which is almost completely dewaxed. To remove essentially all of the remainder of the wax, you can filter it through a double layer of coffee filters. I would recommend using a 1 lb. cut for this because the filtration process is slow and you may need to change filters part way through the process. You will be surprised how much wax is in even a purified shellac like Super Blonde. You will be able to apply dewaxed shellac over the existing shellac and then apply poly over the top of that. I would definitely recommend applying dewaxed shellac over the entire surface before applying the poly. If you only do spot treatments, they will be very visible through the poly finish. Poly over dewaxed shellac works very well and is often used when it is desired to enhance the amber cast of a piece of wood slightly. The amount of color change due to Super Blonde will not be dramatic so don't worry about that aspect. If it were Garnet shellac, then that would be a different matter.
Bill
Bill
You said the chairs are going to be outside on a patio. Spar varnish is an excellent finish to use. It is designed for exterior use and it is compatible with shellac.
Good, TRUE spar varnish should indeed work. And it's great at holding off the weather.
However, be wary to get "normal" spar varnish! There are several varnishes that use "spar" in their titles that aren't up to the job, IMO.
I question if polyurethane spar varnish is a "true" spar varnish. Not sure that you can build up enough finish to protect from weather. Poly is often UV sensitive, too. The real stuff is considered by some to be a unique species of varnish. True spar contains phenolic and alkyd resins in tung oil. It's considered a "high solids" varnish, and because it's tung oil that cures more than dries, it can be a slow "drying" finish. The old stuff had kind of a rich warm yellowish cast to it, but it wore like iron! It's also available with extra UV protection in some brands.
Hope it helps.
However, be wary to get "normal" spar varnish! There are several varnishes that use "spar" in their titles that aren't up to the job, IMO.
I question if polyurethane spar varnish is a "true" spar varnish. Not sure that you can build up enough finish to protect from weather. Poly is often UV sensitive, too. The real stuff is considered by some to be a unique species of varnish. True spar contains phenolic and alkyd resins in tung oil. It's considered a "high solids" varnish, and because it's tung oil that cures more than dries, it can be a slow "drying" finish. The old stuff had kind of a rich warm yellowish cast to it, but it wore like iron! It's also available with extra UV protection in some brands.
Hope it helps.
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As the others have mentioned, oil-based varnishes need some mechanical help to bond to shellac, or even to themselves. Thus the caution about dewaxed - not to the point of foolish - shellac and a solvent wipe after "toothing" the shellac with sandpaper as preparation.
Long-oil varnishes, and they can be made with any of the curing oils, are used outdoors because of their flexibility in response to heat/cooling. Traditional oil used is linseed, tung being a relative late-comer to the western world, whereas flax (linen) was grown all over. Virtue of phenolic resin versus urethane with UV inhibiters is a matter of opinion. The modern stuff is probably good enough, maybe better than its predecessor, the first synthetic resin. Key is that the wood will continue to move with water uptake and loss, film will continue to move with temperature variation in opposition, and the finish will probably peel before it yellows and clouds unless it's living in full sun, something I avoid on my patio. That's the reason why you see a lot of plain oil renewables as "finish" for outdoor wood. That'd be my choice.
Shellac, since it does not share a common solvent with resin, is a great choice for trying to seal knots in softwood. Since we turners tend not to use kiln-dried (and resin-set) wood, it's the only countermeasure we have. You can bake your turnings for resin setting, I suppose, but that brings another problem set along with. Just as outdoor use does.
Long-oil varnishes, and they can be made with any of the curing oils, are used outdoors because of their flexibility in response to heat/cooling. Traditional oil used is linseed, tung being a relative late-comer to the western world, whereas flax (linen) was grown all over. Virtue of phenolic resin versus urethane with UV inhibiters is a matter of opinion. The modern stuff is probably good enough, maybe better than its predecessor, the first synthetic resin. Key is that the wood will continue to move with water uptake and loss, film will continue to move with temperature variation in opposition, and the finish will probably peel before it yellows and clouds unless it's living in full sun, something I avoid on my patio. That's the reason why you see a lot of plain oil renewables as "finish" for outdoor wood. That'd be my choice.
Shellac, since it does not share a common solvent with resin, is a great choice for trying to seal knots in softwood. Since we turners tend not to use kiln-dried (and resin-set) wood, it's the only countermeasure we have. You can bake your turnings for resin setting, I suppose, but that brings another problem set along with. Just as outdoor use does.
What about what is called a Marine Varnish?
R
Ron Sardo
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Tom
What type of shellac did you use?
Most (not all) off-the-shelf shellac has wax in it and will not accept any finish over it.
What type of shellac did you use?
Most (not all) off-the-shelf shellac has wax in it and will not accept any finish over it.
I use a blonde dewaxed shellac I buy at Woodcraft all the time as sanding sealer. After a light sand I have no problems with Wipe on Polyurethane going over the top of it. Like Michael said, a light sanding will probably take care of the problem.
The shellac I used is called "Bulls Eye Shellac" It is made by Zinsser. According to the can it is pure shellac and alcohol.
Thanks for all the suggestions.
Thanks for all the suggestions.
Tom Lewis said:
Tom, That is the old "classic" shellac that has a relatively high wax content; great for luthiers and pen turners, but not real great when it's going to be topcoated with other coatings. So, if you don't want to waltz down the path of mixing your own from the myriad of choices between Ultra Pale and Dark Garnet dry shellac, just use Zinnser's "SealCoat" which is a proprietary processed 100% dewaxed shellac that is reported to have a 2-year shelf life. A note of caution, however, in that in a recent conversation with a prominent waterborne finish manufacturer, I was informed that SealCoat "likes" to crack under waterborne finishes, especially the acrylic lacquers. It may have something to do with the age of the shellac mix that's used so be sure to check the can for its "born-on" date and guide yourself accordingly.
m
R
Ron Sardo
Guest
Tom Lewis said:
Well, you need to remove the shellac if you want to use an outdoor finish. Not a fun job, but not hard. Some may argue that you can get away with just applying SealCoat over Bulls Eye Shellac, but the amount of effort of removing shellac isn't that much compared to starting all over from bare wood if your final finish starts cracking or peeling.
Go to one of your big box stores and pick up a can of denatured alcohol (DNA). Work in a WELL ventilated area and wipe down the chairs with a rag soaked in DNA. It comes off easily and you may need to lightly sand when you are all finish. I should mention, DNA is highly flammable, so be smart.
Then reseal with Zinsser SealCoat. It might be a good idea to thin the Sealcoat with DNA to a 2lb cut. The directions are on the can. Once dry, you can apply a spar varnish. Several thin coats work better than thick coats.
BTW - pure shellac normally has wax, SealCoat is a shellac that had the wax removed (dewaxed).
Good Luck, and work safe.
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