I repair furniture. Sometimes I try new methods because the customer requires something out of the ordinary. In this case, a small table needs finish work but the customer says she would like to keep the old look. My problem occured on the second job of this nature for her. The first time, no problems. 1) I cleaned the old finish with water and smoothed it with steel wool. 2) I wiped an oil base stain over the whole surface and let it dry for two or three days. It was no longer sticky. 3) I successfully sprayed one coat of 1 1\2 lb. shellac, not too thick. 4) The next day I sprayed the second coat (low humidity, warm weather) and a portion wrinkled. My only guess is it needs to dry more. The can says recoat in one hour. What must I do to avoid the wrinkles?
From contributor C:
Just so I am clear on this, you say you wiped an oil based stain over an existing finish? You then applied shellac over the oil based stain? It sounds like the oil based stain never had a chance to get to the wood. Therefore, it just sat there waiting to be wiped off without any way to bond, since it was being prevented by the pre-existing finish. Shellac is a great sealer, but it's not going to adhere to the stain. The shellac was prevented from adhering to the original finish because of the stain. First, use 3 parts denatured alcohol mixed with 1 part lacquer thinner and wipe off the whole thing. Then, depending on what the issue is, apply the necessary steps to correct it.
The big question is after applying the wiping stain, was it left on or was the excess removed? If it was left on, there could be intercoat adhesion issues. If the excess was wiped off so that there was no thick accumulation, then the risk of adhesion issues is much less or not there at all.
What kind of stain was it? That is to say, what kind of binder was the stain? Alkyd? Linseed oil? Another or a combination of drying oils and/or alkyd?
The wrinkling may be from one or two things. The first thing I thought of was how old your shellac is. You did not say. Dump a small amount of your shellac, not too thick, onto a piece of glass. See how long it takes to set up. If the shellac takes a long time to set up, then you have dead or almost dead shellac.
If your shellac is fresh and it dries as it should during your test, then maybe you had a reaction between the solvents in the shellac and the binder in your stain. If you mixed the shellac yourself, then it is likely that you used alcohol as your solvent. This is usually not a problem for stain which has been allowed to cure for the time you said you let your stain cure. But if the shellac has other solvents in it like acetone or something, those strong solvents could be your culprit (if your stain's binder is not made for being exposed to those kind of solvents). The reaction from that kind of incompatibility is wrinkling of the film.
Sand out the wrinkling and shoot again, making sure that you have fresh shellac which does well on your piece of glass. Make sure that you are laying down a light coat, no thicker than 3 wet mils (that's plenty thick).
Let's say that at some point in the history of your item it was touched up with a padding lacquer. Some padding lacquers do not tolerate topcoats. Mohawk's Rapid Pad is just one example of a padding lacquer which will accept more padded on material but not a sprayed on material (padded on material is less than a wet mil while the sprayed coating will certainly be more than a mil and it will wrinkle).
Or let's say that a material like Burn in seal was used to spot spray over a burn in at some time in the history of this item. Burn in seal reveals itself when it is under lacquer topcoats and I have never tried shellac over burn in seal, but my hunch is that it will still reveal where it is and where it is not. I stopped using burn in seal over my burn ins many years ago and I have not gone back. There are other ways of sealing in a burn in to prevent the burn in material from shining through (Mohawk's Finish Up followed by Mohawk's aerosol will do a good job of coating that burn in without shine-through).
Sometimes a lacquer coating just dies. The plasticizers have evaporated out, maybe, or the resin just has lived its life as a knitted film. The solvents from your overcoating will disturb the dead film enough to wrinkle. The workaround is to spray really light coats which dry fast until enough new finish is built up on the old. Then, you can overcoat with a decent wet coat.
First, you cannot put this material on too thick. Orange peel was quite a problem for me. I had to squeeze my fan down, and do 1 wet pass at a time. I usually apply 2 passes perpendicular to each other. This was too much material. I had better luck with more successive coats.
The second thing was quite strange. When my helper sanded too close to the original coat, we would get wrinkles as you described. As he would block sand, he would quit just before he burned through. When I shot this with another coat, we would get wrinkles. I am not sure why this happened, but we did not use anything between these coats. We would knock down the wrinkles and the following coats would not effect the previous finishes.
I am not trying to explain how or why, but just give our experiences. We did not have adhesion problems, and the successive coats blended with the rest. If you are not having adhesion problems, sand and recoat. But I would certainly rule out any problems with the stain.
Most wiping stains that you buy as wiping stains have a binder in them. That goes for the DIY materials as well as the commercial-grade materials. Fresco colors are examples of pigments with no binder and no liquid medium. They are just powders.
I do not think that it is enough to say that wrinkled shellac is exhibiting an adhesion problem. If what you are calling "wrinkled" another finisher will call "bubbled", I can see that. If what you are calling "wrinkled" is nothing more than really bad orange peel, then it is not an issue with adhesion but an issue with flow out.
After the wrinkled area cures, if it flakes off (cellophanes or peels like sunburned skin), then there is a problem with adhesion. After all, it is not adhering and it did not become part of a continuous film with the coat under it as it should have since shellac is an evaporative coating.
If the wrinkled area is not flaking off, but it is only grossly disfigured, then it is not an issue of adhesion or lack of adhesion. It is an issue of either an incompatibility or very bad flow out.
Since the solvent used was denatured alcohol, maybe that is the culprit, who can say? Perhaps the denatured alcohol had a high water content - would this cause such a problem? Maybe the methanol content of the denatured alcohol was too high. Methanol is an alcohol that has a high solvency. Ethanol has a lower solvency.
Old shellac dries slowly or not at all. And if the shellac is old or somewhat dead for whatever reason, maybe that area of finish was not ready for the subsequent coat it received. Testing the shellac as I described will tell whether the shellac was good or not. Looking at the date on the can is not good enough.
Anyway, the workaround is that if the area is not flaking off, sand it smooth and recoat but go lightly. If there is a problem with bubbling or flaking, then sand this area out and get rid of all the flaking and again, build up new shellac, but lightly.