Bubbles in lacquer topcoat

      What causes bubbles in lacquer topcoats? September 26, 2000

I run a medium-size refinishing shop and from time to time we get small bubbles in our final coats. We use regular sanding sealer with two or more coats of various nitrocellulose lacquer.

Retarder, different dry times, stripped pieces or virgin woods don't seem to have a connection.

Bubbles in the final topcoat can be caused by many things. Wood species also plays a part in this problem. More porous woods, such as oak, mahogany, and walnut, for example, will tend to bubble more than less porous woods or woods with tighter grain patterns.

Most causes for bubbles can be directly related to stain not being properly dried prior to applying sealer and topcoat. What happens is that the stain is still releasing solvent after the sealer and topcoat have been applied, thus causing bubbles. The solution, of course, is assuring the stain has had sufficient time to dry before topcoating.

Another cause is using a sealer and/or topcoat that is too high in viscosity. A lacquer or sealer works much better at 17-18 seconds viscosity in a No. 2 Vahn viscosity cup. Add a good grade of lacquer thinner and retarder in the right proportions to achieve the proper viscosity. You want just enough retarder to keep the lacquer open long enough to flow and rewet overspray when spraying a piece, but trying to achieve this with lacquer retader alone will not fix the problem, and will sometimes make it worse. Your ratios for thinner and retarder will vary depending on weather factors including humidity and temperature. This will be a trial-and-error process on your part to make it work.

I will add that a nitrocellulose lacquer and sealer finish is more forgiving than a conversion varnish or similar finish. Bear in mind also that your stripped pieces may still retain some of the wash solvents down in the pores of the wood if you are not letting them dry long enough.

There are many things to consider with this problem. Look closely at what I mentioned above and see if any of these causes may at the root of your problems.

From the original questioner:
Thanks for the quick responses and advice. I have tried all the solutions in the past but without any consistent results.

I know that as temperature and humidity go up it seems to happen more frequently. You're right that retarder alone increases the problem. I have had to re-strip many a piece before giving up on that as a solution.

If any of you have any more advice, I would like to hear it. If it makes any difference, sometimes I can get satisfactory results from spraying the tops on their sides (only happens on horizontal surfaces) letting them de-gas easier.

I'll try some retarder in with my regular thinner on the next tops and see if I can get the right combo.

Make sure that you are using a reducer that is of the same brand as your finish. You didn't say if you were stripping or not; if so, make sure all of the methylene chloride is out of the wood.

I often find that thinners and reducers can cause the problem you have described. A "hot" or fast-drying thinner with the incorrect air flow (fast or excessive) can cause the thinner to flash off too fast, leaving solvents under the dried film with nowhere to go, thus causing bubbles or pinholes. Viscocity means everything in situations with minimal air flow.

Not knowing what products you're using other than nitrocellulose, I would suggest that you keep your lacquer warm and off the concrete to lower the viscosity, and if any reducer or thinner is needed, I suggest you use it sparingly.

Also, while spraying, make sure you apply a wet coat, and check the air coming out of your gun. Too much can cause air to get trapped under your finish.

May I ask if you are using a lacquer that has lot of acetone in it? I did new and refinsh work for over 20 years and know what you are talking about.

What are you using for a retarder? Also, what is your brand of finish (if you want to tell us), or are you in a state that requires low VOC's and you have one of "those" lacquer formulas that are a little temperamental?
Bob Niemeyer, forum moderator

Since you said you have the problem on just the tops and not the sides, what I am about to recommend should help a lot.

When spraying the top of a piece, many people have a tendency to apply too much material. Lighten up on the top some, don't cross-coat it or overload it with material. Just spray a medium wet to wet coat and that's it. Allow all coats to flash off or dry in between and also take the advice mentioned previously about reducing your material. Hope this is of some help.

You say that you see it more when it is humid; are you sure that it is not fiber stand you are seeing?

There is bubble relaxer out on the market. Contact your supplier or look for more suppliers on the 'Net. I have used acetone to get rid of the bubbles and that seems to work.

Finally, check the air pressure of your gun; too much will often cause bubbles.

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