Shellac over Casein — Bubbling and Adhesion Problems
Finishers try to troubleshoot a performance issue with an unusual custom art finish: milk paint and shellac on furniture. February 13, 2009
Question
I am an artist doing casein paintings on wood furniture. I do sell my work in galleries so I checked the professional box. Casein is similar to acrylics except it must be coated with shellac since it is not water resistant by itself. It could be a long post if I explain why I prefer casein and shellac, so I won't, I'll just state my problem and hope someone knows an answer. Shellac blisters if exposed to heat, even just sunlight coming in a window on a hot day. I have had a couple pieces ruined that I had on my patio in the sun. They were just out there for a couple hours - I don't keep them out there. Those pieces did have several coats of shellac. I'm wondering if I just put one coat of shellac to seal the casein, then apply a more durable finish coat if that will still blister or was it because of too many coats? I have done several tests with mixed results. I know shellac is commonly used as a sealer under other finishes. Are those pieces subject to blistering because they have a coat of shellac? I sure hope I can find someone with knowledge about this problem.
Forum Responses
(Finishing Forum)
From contributor G:
How long did the casein cure before you shellacked it? How many coats did you put on with how much dry time between coats? Was it alcohol or water-based shellac? That degree of blistering doesn't sound right.
From the original questioner:
The casein sat at least overnight, but some areas of my art have no casein, just either natural wood or dyed using Lockwood's water soluble dyes. I coat the entire surface with shellac and then sometimes paint some more art and shellac again, up to four coats total. I use either shellac bought mixed or unwaxed from flakes, and it seems to act the same.
The shellac dried several hours at least between coats. I have heard of shellac blistering when exposed to heat before. I understand it used to be used as a floor finish years ago. The only shellac I have ever seen is alcohol based. I cannot put water based clears over casein, they will smear it. I have used other solvent based clear over casein but I like the way shellac and casein work together. I do always put a more durable clear over the shellac but in my tests it seems to make no difference. If there are coats of shellac overcoated with something else or not, it will blister. In my tests I have seen that just about any finish will blister if heated enough.
I'm using alder, poplar, pine, or cedar, all dry wood. I guess my bottom line question is: if I limit the shellac to one or two coats will it blister if exposed to heat? I'm just afraid of putting a lot of hours into a piece, selling it and having a disappointed buyer. I know an artist who does landscape painting in casein and coats of shellac, but since they hang on a wall they are probably never exposed to direct sun or heat. I'm a semi-retired sign painter trying to find a new direction and enjoying it. Thanks for any input. I appreciate all the knowledge here.
From contributor P:
If you're using shellac as a sealer (vs. a standalone finish), you should use dewaxed shellac. Zinsser SealCoat is one example of a dewaxed shellac. If you use shellac that is not dewaxed, you can expect adhesion problems if you try to topcoat it with something else. Not sure if that's the cause of your problem, but it could be. Also, shellac flakes and premixed shellac have issues with shelf life. If the shellac is too old, it won't cure properly. If it still feels gummy an hour or more after application, it's probably expired.
From contributor S:
You could also be applying too heavy a cut of shellac. If the coat is too thick it may not be drying enough before you move to your next finish step. I would use 1 to 1.5 pound cut for what you are attempting.
From contributor G:
I suggest that no art painting, especially ones on wood, should be exposed to direct sun, heat, or for that matter, water either. Shellac is not that great at protecting from water anyway. I understood that casein, cured for six months or more is fairly waterproof by itself. Is this incorrect? Have you experimented with other clearcoats? Varnish is an option. So is Weldbond.
From contributor D:
I'm missing something. Finishers, myself included have for years, not centuries, been using shellac as a finish alone. French polishing, after all is the application of microscopic layers of shellac. I have never encountered a bubbled shellac finish from simple sunlight exposure. I feel there must be an adhesion issue between the casein and the shellac. You don't mention what exactly bubbles, is it just the shellac and the casein is unaffected or does the casein bubble along with the shellac leaving bare wood exposed?
From contributor R:
It's time to get out the magic magnifying glass again. You need to do a little forensics. Look very carefully at the blistered areas through a powerful magnifying glass or 100x microscope. Where is the failure? Like contributor D said, is the finish failure occurring at the bare wood, the casein or both. That info will help in determining what the problem is.
From contributor S:
It is heat that will blister shellac. UV light alone is not the actual cause, it has to cause enough heat for the blistering to form. Shellac is thermoplastic. Different grades and point of origin have a varied melting point so I can't give you an exact temperature rating, but an item left out in the Arizona sun clearly reaches it on your pieces.
I suppose the thinner your shellac coating before you overcoat may reduce the size or more accurately the visible appearance the eye can see, but no matter how thin if it heats up enough there would be blistering which you could see with a microscope. Make up some sample boards of different mil thickness and run some tests to find out what parameters will work for you.
You could look into adding another resin into your shellac, one with a higher melting point to see if it will help stabilize the coating a bit. But as stated by an earlier post, your pieces that are polychrome probably should not be exposed to an environment that will cause the problem anyway.
From the original questioner:
The entire finish, shellac and casein appears to be lifting from the wood in bubbles from microscopic up to dime size. It is also lifting where there is no casein. I believe the shellac penetrates all the way through my thin casein coats to the wood beneath. A problem with a lighter cut of shellac is it then requires an extra coat to form a waterproof barrier over the casein. Maybe I need to find the bare minimum amount of shellac to use.
Yes, casein does become water resistant after a couple of months but of course that’s not practical for me to wait. And, yes there are alternatives to shellac to seal casein but they don't work as well (other than they don't bubble).
From contributor G:
Another thought - are you using kiln dried wood? It almost sounds as though there may be volatiles gassing off under the warmed, softened shellac.
From the original questioner:
I think I'm narrowing this down. The piece with the worst blistering may not be really dry wood. The kiln dried I use has only blistered when I get it extremely hot, hotter than it’s likely to ever get normally. It’s difficult to do an accurate test in the sun this time of year, so I may be overheating in my tests. I am going to print on the bottom side "keep away from heat".
From contributor T:
That said there's nothing wrong with shellac for your purpose. There are a couple of other natural resins that would also do the job and a large number of synthetic resins as well. Shellac is thermoplastic and will melt and flow at higher temps. I've also had it cuddle but I've never seen it blister. It sounds to me like your problem is adhesion between the casein and your substrate. The simplest solution is to lay down some shellac before you paint, paint, and then cover it with shellac. The first shellac coat will improve the adhesion between the substrate and the casein. I would also let the casein dry as long as possible before covering it with shellac.
Virtually all finishes auto-oxidize and shellac is no exception. You can minimize the effect of UV (but not heat) on shellac by adding a free radical scavenger.
From Paul Snyder, forum technical advisor:
If you're going to use milk paint and get good results, you just need to make a couple changes. First, do not layer the paint between coats of a clear finish. That's just like applying really heavy layers of glaze between coats of finish.
Second, switch to a more flexible sealer for the final, protective coat. Shellac is very hard/brittle. Oil-base varnish is very traditional though an acrylic would avoid the yellowing that comes with the varnish.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor B:
The likely cause of the blistering seems to be inadequate drying time, especially if thick multilayered coats have been applied. The alcohol in the shellac takes a while to evaporate. When exposed to excessive heat the accelerated out-gassing causes blistering. It should be noted that noon day sun will destroy a shellac finish even if it has been dried correctly.
