Troubleshooting Bubbles in Pre-Cat Finish on Oak
Finishers offer theories and propose solutions for bubbles that appear during drying in pre-catalyzed finish. December 29, 2008
Question
I am shooting a fast drying pre-cat and every once in a while, it bubbles up as it is on the drying rack. Kind of like air is trying to escape through the pores of the wood. Today is one of those days. Any tricks on keeping it to a minimum? Light coat helps some, and I am adding a little retarder to slow down the lacquer.
Forum Responses
From Professor Gene Wengert, Sawing and Drying Forum technical adviser:
Indeed, it is the air in the pores being heated, expanding and coming out of the wood, except the finish is too thick to flow back smooth. Heat the panels before finishing and do not let them get hotter can be a solution.
From the original questioner:
I have had some door panels soaking in the sun this morning and I still got them darn bubbles. So far, shooting a very light coat and letting it dry 10 minutes or so before shooting a final coat has helped some, but still every now and then... Maybe I should be shooting late in the afternoon instead of earlier in the day? I'm in the Houston area if that helps any.
From Professor Gene Wengert, Sawing and Drying Forum technical adviser:
Avoid any heating after the finish is on.
From the original questioner:
Thanks Gene. I am putting them out for a sunbath and then when I shoot them, I place them inside on a drying rack. I know they have to be cooling down after they are shot. I think part of my issue is trying to shoot them heavy and get it done quick. Slowing down and shooting two light coats seems to be better. I have used the sunbathing trick before, but even this time, it didn't want to work.
From contributor T:
That is solvent pop. The retarder should help by slowing down the drying time, preventing the finish from skinning over before the solvent can flash. How much retarder are you adding? Can you maybe push it up anymore? Also, spraying earlier (when it's cooler) would be better than later in the day when it's warmer.
From the original questioner:
I can't conceive of it being solvent pop, as it won't do it on the plywood or even a piece of maple. Just the oak hardwood. I hate to add too much retarder. It's not good for the finish and it turns satin into semi-gloss for some reason.
From contributor T:
Yes, but the grain pores of the ply and especially the maple would not be as deep and/or defined as the oak hardwood. I have never had solvent pop on anything but oak hardwood.
From contributor R:
It would help to have your entire finish schedule. I find that a wash coat of sealer or two as the first step after my stain will eliminate most of the problems.
From contributor B:
Typically, bubbles are caused by:
1) Material drying too fast so that the bubbles cannot pop and flow out.
2) Excessive air movement (like near a windowsill).
3) Material is too high in viscosity to allow proper flow into the open grain of the oak, walnut, etc.
4) Glue in veneers causing off-gassing.
5) High moisture content of the wood.
I have never recommended retarding a seal coat since those slower solvents tend to be "trapped" in the seal coat or in the wood and could cause a curing problem with the final coats (not getting hard enough).
I would recommend thinning your seal coat 15-20% and seeing how that resolves your bubbling issue. If that gets you close, but not enough, then retard your finish, and only then with a straight-solvent retarder, not a blend or slow lacquer thinner - those are not efficient enough to facilitate proper retarding of the dry time.
Use a good quality lacquer thinner. Too many cheap ones out there are high in moisture content themselves, and that could contribute to your problem. The wood's moisture content should be less than 9% for proper finishing. I know most shops do not have a moisture meter, but your lumber supplier should.
From the original questioner:
Thanks guys. As to the finish schedule, it is spray, then wipe Old Masters penetrating stain. Let dry overnight. It is a very fast drying stain also. Second step is vinyl sealer, which is the recommended sealer before the pre-cat I use (Gemini). The vinyl sealer is very thin already, and I didn't add anything to it. No bubble trouble there. Only on the pre-cat.
Then scuff sand the nibs and two coats of pre-cat, thinned about 20% to make it flow better. It is pretty thick otherwise.
I went back closer to dark, and I could shoot heavy coats with no trouble at all. It was during the heat of the day that all the trouble was taking place.
From contributor O:
At the risk of being poo-pooed, I will suggest that the surface tension of your finish is not quite low enough. The surface tension of a finish must be below the critical surface tension of the wood in order to get good flow out. Higher surface energy causes a lower critical surface tension. Unfortunately surface energy likes to concentrate at sharp edges - like the edges of large oak pores - and warming the wood simply kicks the energy in the pants, making the problem more pronounced. Finish that is applied thick enough to flow just doesn't want to flow over those pores if the finish's surface tension isn't low enough. Thin coats don't flow, so you don't see the problem.
Sounds to me like you're right on the edge. You've solved the problem by spraying when it's cooler. You could also eliminate the problem by adding a very small amount of flow enhancer (aka fish-eye eliminator) to reduce the surface tension of the finish. (Retarders and solvents do not reduce the finish's surface tension enough, and no - flow enhancer will not contaminate your lines forevermore.)
From the original questioner:
Very interesting thought. Will fisheye eliminator work in pre-cat lacquer? I used it back in my car painting days. I never considered it on this type of job. It doesn't look like fisheye, though. Fisheye separates from a spot and causes a crater. These are bubbles. I'm scratching my head on this one, but I am willing to give it a shot.
From contributor O:
Fisheye is caused by a local miss-match in the critical surface tension of the wood surface. In the contaminated spots, the CST is much lower than in the uncontaminated spots. So the finish doesn't want to flow as well over the contaminated spots as it does over the uncontaminated spots and you end up with a crater. To deal with it you must add something to the finish that lowers the finish's surface tension. That's what fisheye eliminator does. It just lowers the surface tension of the finish to get it below the lowest CST of the surface.
From contributor B:
For the dreaded "Don Ho Syndrome" (tiny bubbles), I've just added a little retarder to my mixture and applied lighter coats until I had a decent surface that allowed me to apply a thicker coat. Only once have I had a sheen difference and that's because I added too much retarder to my material.
From contributor S:
Simply put, wood retains heat when put out in the sun like you mentioned. By not letting it cool down, you will speed up the dry time on your lacquer.
