Troubleshooting Bubbles in Spray Polyurethane

Advice on eliminating bubbles from a poly finish on a veneered table. August 17, 2009

I have a large conference table that I built using MDF and 22 ml paper-backed maple veneer. The veneer supplier recommended that I use a polyurethane or "flexible" finish, discouraging the use of lacquers and similar "harder" finishes for fear of cracking.

I sprayed an initial coat of urethane as a sealer, sanded the top with 220 and tacked it off before applying additional coats. I ended up with little bubbles in the finish which has become a nightmare. I contacted the mfg of the finish and they told me that I should spray lighter coats. I started back to spraying samples of similarly prepared pieces. I am still getting bubbles, with the lighter coats the bubbles are much smaller, but still there. I am using HVLP system in a fairly controlled environment. I have tried using a reducer, and the mfg said that that is widely done, although they could not officially recommend thinners. Does anyone have any suggestions or experience with this type of thing? At this point I would never think of using a urethane product again.

Forum Responses
(Finishing Forum)
From contributor S:
I am a finisher not a cabinetmaker, but let's start at the beginning just to cover the possibilities. Did the mfg suggest a flexible finish because you were using a contact cement to put your veneer down? If not, what kind of glue did you use? With 22 ml veneer you shouldn't be having a glue bleed through problem but if you used a contact cement, you generally shouldn't use a hard finish.

Are you using an HVLP conversion gun or a turbine unit? If you’re using a turbine, how long is your hose from the unit to the gun? What is the name and mfg of the product you are using? How many coats did you put down, at what wet mil thickness, and at what coat did the bubbles first appear and how much dry time between coats? What reducer did you use? How large is the table?

I assume this is in a shop, what is the environment (temp, RH, etc.)? Do you think there is room to sand back to before the bubbles showed without breaking through your sealer coat? Depending on your answers I may have suggestions that will help solve the problem.

From Paul Snyder, forum technical advisor:
Are you spraying catalyzed polyurethane or oil-base poly? What are you thinning it with? You can avoid the bubbles by thinning the finish with a medium to slow evaporating compatible solvent and applying it in thin coats. Some finishes are a lot more finicky than others and you have to thin quite a bit to prevent the bubbles. The bubbles that are already in the finish will have to be sanded out completely or they will continue to be visible with further coats.

From the original questioner:
I did use spray able contact adhesive. I am using an air HVLP air gun and I have tried several different guns, gravity, pot, and suction. I am not sure the exact mil of the finish I applied. How would that be able to be measured?

I did not notice a lot of bubbles on the sealer coat, mainly on the second coat and I went into repair mode. I have been able to sand most of them out and the table appears ready for a good top coat. I am hesitant to shoot it until I have good results on a sample or two. I am a small one man shop and I do not have a fantastic spray booth, but generally have very good results with lacquers. I have the ability to control humidity and temperature. The conditions are fairly consistent in my shop. I am spraying Minwax fast drying time which takes about six hours to re-coat. I waited at least that long before between-coat sandings and re-application.

I have tried a couple of other brand as tests and the Minwax seems to provide the most promising results if I can avoid the bubbles. Can you recommend another product? I read somewhere that there may be a problem with shooting urethane in a gun that has been used for lacquer. I am religious about cleaning my spray equipment. The table is 6'x8' oval 2.25" solid. I used paint thinner and mineral spirits as a reducer with similar results.

From the original questioner:
I used an oil based poly, Minwax, and I tried a few others, but his seemed to lay down the best, with the exception of the bubbles. I tried thinning with paint thinner and mineral spirits with similar results. I would like any suggestions on other finishes and reducers if you have any.

From Paul Snyder, forum technical advisor:
Did you try spraying with no thinner? Sometimes that actually works the best. If you did spray it un-thinned and had the problems try some samples with different amounts of thinner (mineral spirits). Start by thinning 15% and spray out a good size sample. If you get bubbles, add 5% more thinner. Keep doing samples and adding the thinner in 5% increments until you don't get bubbles any more. You'll have to apply a couple more coats to get the same build since the finish is thinned so much.

Oil base varnish, including polyurethane, isn't really considered a spray finish but can be sprayed. In a spray finish, conversion varnish or catalyzed polyurethane (2K PU) would offer a lot of durability and dry and cure more quickly than the oil-base varnish. Either of these would be a good choice as long as you observe all the safety precautions from the MSDS sheet.

From contributor A:
As a general rule of thumb oil based poly's and varnishes should be reduced with Naptha for spraying. When brushing in colder weather Naptha is a good choice. Mineral spirits is the appropriate brushing reducer for most applications.

Naptha (VM & P Naptha at the hardware store) has a faster evaporation rate than paint thinner or mineral spirits. Many of the proprietary spray reducers are mostly Naptha. The hardware store finishing products have all had the voc's taken out of them. I suspect the Minwax poly is too thick for your under powered turbine. Even with a compressor gun we reduce at least 10% with Naptha. The sealer coat we reduce 25% or more.

From contributor S:
I'm not fond of Minwax products so I don't know anything about their poly. The only hardware store brand I ever used was Varathane and that was probably 18 years ago so I can't vouch for their formulas now. But I used to do a lot of onsite finishing and the Varathane worked well for boardroom tables and desk tops. I would roll them on and roll out the bubbles and they come out perfect. There are tricks to rolling but it is easy and works well. You have to use a short nap 3/8" or 1/4" Mohair roller. If that interests you let me know and I'll explain the technique. If your poly is ready to be sanded it is dry enough for recoat. You will have to remove all sign of bubbles before recoat.

If you plan to keep spraying:

You are using a HVLP with a compressor right? Check your lines for moisture and your pressure at the tip. Don't worry too much about your mil thickness, as long as you lay it on as a full wet coat and have no runs or curtains you should be ok. You can buy a mil thickness gage at an auto body paint store if you want to learn to read mil thickness.

Remember you can mix your reducers together also to get a better flow and evaporation rate to the way you want it, Naptha and mineral spirits. Try spraying on a piece of glass to check how it lays out. If it lays out nice there you are good to go. If you still have bubbles there you still have a problem.

You can't use a hard finish like CV or 2K because you used contact cement to lay down your veneer. The contact cement is not a hard dying adhesive, so if you coat with a hard finish it would be subject to cracking in the future. That is why they told you to use a flexible coating.

From Paul Snyder, forum technical advisor:
CV and PU often have good flexibility, though some don't. You can check with your supplier to see if they have ASTM D522 data on the finishes you're considering. Take a look at the first paragraph in the link below for an overview of the properties of PU including flexibility.

From contributor B:
I've had the same exact problem in the past. I would have to agree with thickness of the first coat or two is (especially the first) the culprit. I've had it crop up with both sw, cv, and h2o based finishes. The common thread has always been a too thick of a first coat. I've even had it happen on plywood end panels/door panels especially thinner skinned ply's like cherry and mahogany.

From the original questioner:
I was able to work through this with your help. I found that thinning with Nnaptha, and applying several very light coats allowed me to build the finish up to a point where commercial compounding and polishing would level the finish out and look pretty good. With the light coats the finish does not get a chance to flatten out as much as I would like, but is does polish out fairly. I must say I would rethink this project from beginning to end and avoid the urethane at all costs. This forum is a great tool and I hope to help others in the future as well.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor D:

Chemcraft has a product called D-Dur that I use. It lays down well and does not seem to bubble.