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.
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.
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.
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.
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.
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.
Comment from contributor D: