[OP]
Member
I am hoping someone can help with a question about a shellac/poly finish. I recently finished putting on a coat of shellac and allowed it to dry for 24 hours. Then I used an HVLP gun to put a coat of polyurethene. I think I put too much on the top of the dresser and now its dryish by very soft. Is this something that will firm up over time or will I need to start over? I wont put this furniture into use for about 5 months so i have time to wait if needed.
Member
Where are you and how humid is it?
Member
Its a bad idea to put a thick coat of poly on for the reason you are now finding out. The oil in your poly, which is usually linseed oil, needs thin layers to be able to cure properly. The thick coat of poly might dry, but it will most likely be softer than if you'd put multiple coats on. Next time try a wipe on poly instead. It's hard to mess that type of finish up.
Member
First off shellac (wax free alcohol base) is not a recommended seal coat before applying oil base poly (a sanding sealer would have been a better choice). Actually Zinsser and other brands say that right on the can.
Second, did you sand it at all before applying the poly?
I think what you have could be a couple things of which the poly not being compatible with the shellac under it or your poly was sprayed to thick and not thinned enough. A coat of Poly usually film dries to the touch within the hour but takes many hours/days for full cure after 2-4 thin coats. If one coat is tacky very long after like the next day you have a problem and when it does dry hard to the touch it could still suffer adhesion problems later.
Matt Tawes
Chesapeake Woodcraft
Moderator
It's pretty hard to put on a too thick coat of poly with an HVLP gun. Unless you REALLY over shot, then your coat would be quite a bit thinner than even a single brushed coat, which is not 'too thick'.
The best thing to do now is to give it some time. You've got a good chance that it'll cure up fine. I'd put it in a relatively dry area with temps between 70-80degrees and not too much direct sunlight. You want to give it time to cure in its own way and don't want to 'help' it along with excessive light or heat...
I've had solvent finishes take up to 3-4 days to dry right depending on the oil content and the temp/humidity.
As for waxed shellac under poly. I wouldn't do it normally, but many have reported success doing it. So, don't whip out the sander yet.
Be patient!
Member
Huh?Originally Posted by Matt Tawes
From a can of Zinsser Bulls Eye Shellac (spray):
"It can be topcoated with any interior paint or clear finish..."
and
"Because Spray Shellac is 100% wax-free, it can be used as a sealer under polyurethane and other finishes."
From a can of Zinsser Bulls Eye SealCoat (gallon):
"Because SealCoat is a 100% wax-free shellac, it's guaranteed to be compatible with ALL clear finishes, including oil-base polyurethanes, water-base polyurethanes, lacquers, catalyzed finishes and epoxy as well as traditional varnishes and shellacs." [bold and caps not mine]
Moderator
It's waxed shellac that's not compatible: Zinsser Bullseye.
The aerosol stuff and Sealcoat are both dewaxed, so they're safe 'primers'.
The aerosol and Sealcoat are both blonde.
Member
I guess you're correcting my advice with this.
No problem I should have clarified and been more specific I suppose... Sealcoat/sanding sealer and actual shellac are 2 different products and it does on many mfr. instructions state "not recommended" under poly (non poly varnish vs. poly over shellac is different as well). Here's the actual Technical data link for Zinsser where it clearly states not recommnend: http://www.zinsser.com/pdf/TDB/be_shllac.pdf
"Bulls Eye Shellac is not recommended under polyurethane
finishes. To seal wood before finishing with polyurethane use
Bulls Eye® SealCoat™ Universal Sanding Sealer."
I never said it does not work but it can present compatabilty problems. Zinseer and other shellacs/sealers and products are excellent and work universally well with most finishes.
I would not mislead anyone with my advice if it were not founded and pretty true to past experience but I'm not a know-it-all either. I've been a profesional woodworker/finisher for 23yrs. so I just offered my opinon,advice and experience which I thought would help.
Howard Acheson will probably chime in as well and I'm sure he'd agree with me to some degree and he's the finishing guru.
Last edited by Matt Tawes; 06-17-2009 at 12:46 PM.
Matt Tawes
Chesapeake Woodcraft
Moderator
What exactly is IN a "sealcoat/sanding" sealer anyway?
Techincally, the first coat of any product you apply to wood could be considered a "sealer". It stops up the pores.
All a sealer has added to it is a finish (shellac, for example) and some mineral soap (zinc stearate) to act as a lubricant to make sanding easier. Varnishes, water base, and most lacquers are difficult to sand so you'll have "sealers" available for those finishes. Shellac, polyurethane, and some lacquers that have this mineral soap included are easy to sand so you don't need a "sealer".
Now shellac under lacquer is great (even waxy shellac is fine) and is often used as a first-coat or sealer.
Usually, polyurethane doesn't bond well to finishes that contain mineral soaps or to waxy shellac. Dewaxed shellac, however, should be fine.
Member
>> What exactly is IN a "sealcoat/sanding" sealer anyway?
Zinsser Sealcoat is pure dewaxed shellac and alcohol as the solvent thinner. It's nothing special as a sanding sealer. Adding "sanding sealer" to the label is just an attempt to broaden the product base. It's really not a very good sanding sealer as it has few solids to fill the grain and it is fairly hard to sand. However, Sealcoat is a good standalone shellac finish and it's an excellent barrier coat between incompatible finishes and stains.
Both the Zinsser Sealcoat and the Zinsser spray can shellac are standard dewaxed shellac and any finish can be applied over them.
On "sanding sealers" in general, most are a lacquer based finish containing fillers and stearates to improve grain filling and sanding. The stearates are the problem. They are a form of soap to ease sanding and finishes other than lacquer do not adhere well to them. They are also quite soft and reduce the water vapor resistance of lacquers. Lacquer based sanding sealers should only be use under NC lacquers. Two part lacquers require their own sanding sealers and they should only be bought from the manufacturer of the two part lacquer you are using.
In general, for those not doing production spray finishing, sanding sealers are not necessary.
Howie.........
Member
Matt, the only reason I brought it up is because in your post you said "wax free alcohol-based", which I took "wax free" to mean "dewaxed". Then the product to which you linked after I posted is traditional shellac, containing wax. And no, I wouldn't recommend that under polyurethane, either. As for the SealCoat, I've used gallons and gallons of it on natural/stained trim packages, and as the first coat on tens of thousands of square feet of hardwood floor since it came out. If there were a problem with it under poly, my phone would be ringing off the hook, and I would be run out of Indiana for sure.
Perhaps you simply mis-typed it. It was just one of those things that jumped out at me.
Member
Hey no harm no foul Jason, I don't get into pissing matches over stuff just trying to give an informed opinion on what I thought might be the issue with Clints poly being tacky.
Shellac is just such a common misunderstood finish most the time anyway for folks with waxed, dewaxed water base and some that are just plain lacquer, those like Zinsser shellac (small wax content) and their sealcoat (dewaxed). Actually I use Target WB lacquer as a sealcoat/sanding sealer most often anyway if I thnk I need it unless a high solids sealer is needed.
I use poly over shellac too sometimes (but over waterborne shellac).
Hopefully Clint gained some info. out of this and that his project turns out and dries.
Matt Tawes
Chesapeake Woodcraft
Member
[OP]
Member
I am in Kansas and it has been very humid the last few days and the Dresser was in the garage. I moved it inside after I posted this thread and the finish seems to have hardened considerably. I will however take this as a learning experience since it was the first time I used an HVLP gun to finish a project. It was great fun but I could have used a little restraint.
I was fairly certain this combination of finish with shellac and poly would work because I used it on a Mug I turned on the lathe and I use it everyday. My overzealous spraying I think is what started the problem but it seems I did not go entirely over the line and things will be ok.
I think the project is safe for now but I am sure that it will need refinishing within a few years due to nicks and dents. Thanks everyone for the helpful insights.
Contributor
Did you thin the poly before you used it?
Many of the OTC DIY products are way too thick in the can. VOC's is the reason.
Use a HARD varnish, not a tough varnish (poly) for furniture. Big box stores don't carry it.
Behlen's Rock Hard Table top, Waterlox, Sherwin Williams Classic oil varnish, Pratt & LAmbert #38 (my fav).
Also the varnish does not contain oil that needs to cure, it's varnish, the varnish needs 30 days to cure. The oil and the resin that were combined to make the varnish are now a new product; varnish.
Scott
Finishing is an 'Art & a Science'. Actually, it is a process. You must understand the properties and tendencies of the finish you are using. You must know the proper steps and techniques, then you must execute them properly.
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