I'm hoping to tap into the experience of others who may have run into something similar.

We've been investigating an intermittent bubbling issue that occurs when spraying a solvent-based transparent conversion varnish sealer on veneered panels. The product is reduced with 10% butyl acetate and catalyzed per the manufacturer's instructions. The bubbles typically appear within 1–2 minutes of spraying and, importantly, we've confirmed they can occur before the parts ever enter the drying oven.

Over the past 6-8 months we've systematically evaluated numerous process variables, including mixing method, application method, oven versus no oven, sanding conditions, operators, surface cleaning, and two different solvent-based conversion varnish systems. None have consistently eliminated the bubbling.

Some observations:
• Bubbling has occurred with wet film builds as low as 2 wet mils, although heavier applications tend to produce larger bubbles.
• The issue is intermittent—even adjacent veneer leaves from the same sheet can behave differently.
• The problem has not occurred on solid wood or raw MDF.
• The issue is much more prevalent on 3/4" veneered panels than on thinner veneered materials.
• Bubbling has occurred on multiple closed-grain veneer species, including Select Alder, Knotty Alder, Select Cherry, and Premier Maple.
• We've observed the issue on veneer from more than one manufacturer, so it doesn't appear to be isolated to a single supplier.
• Adhesion testing over both bubbled and non-bubbled areas has consistently produced excellent results.

At this point, we suspect the issue may involve an interaction between the veneer substrate and the solvent-based coating system rather than a single isolated process variable.

Has anyone experienced something similar, particularly:
• Bubbling before oven exposure?
• Bubbling that varies from leaf to leaf within the same sheet?
• A veneer-related issue that wasn't present on solid wood or raw MDF?
• A finishing process that proved more tolerant of normal veneer variation?

I'd appreciate hearing about any similar experiences or solutions that proved effective. Thanks in advance.

What kind of airflow is in that area. Is it possible you are getting a skin over issue and then the bubble forms because of that?

Thanks, Leo. I appreciate the response. I agree that rapid skin-over is likely part of the physical mechanism that allows the bubbles to form. After spraying our test parts, though, the parts are moved out of the spray area and dry in a normal ambient (room temperature) environment, so direct airflow does not seem to be the main driver.

The part we're still struggling with is why the same coating process does not bubble on solid wood or raw MDF, and why it varies leaf-to-leaf on veneer from the same sheet.

One other observation that may be relevant is that applying a solvent-based wipe stain appears to dramatically reduce the occurrence of bubbling. We still experience some intermittent bubbling even with stain, but it is greatly reduced compared to parts that receive no stain. We haven't determined why yet, but it seems like another piece of the puzzle.

Have you seen skin-over issues that only show up on certain veneer leaves or specific veneer constructions? If so, did you find the solution was more related to application (film build/flash time) or to veneer preparation?

No I haven't. The randomness of this happening must be maddening. I really hate these types of issues. If the staining helps maybe do a test with just stain base with no color in it to see if that does the same thing. But that sure would make a lot of extra work on a natural finish. But it's just a test.

one thing that helped me if ur using a kremlin airmix, get the two series tips, like if ur using something like an 09-114 get a 09-112 it has internal plate that pre atomizes the coating before it reaches the tip...has help me enuf also if using a binks, graco, etc aaa, they have the same type of tip, sometimes called a pre atomizing tip

good luck herb j

Thanks, Leo. That's a good suggestion. I had actually been thinking along those same lines. Testing the stain base without colorants would help determine whether it's the stain vehicle itself that's making the difference rather than the pigments. I think it's definitely worth trying. I appreciate the idea.

Thanks, Herb. I appreciate the suggestion. We are using Sames AAA equipment, and our batch booths do currently use the 134.509.114 tip. I noticed the 09.112 is listed by Sames as intended for water-based materials. Are you having success using the 09.112 tip with solvent-based coatings?

you know the factory says a lot of stuff i think just to promote another tip...the best thing they say about the tip is that is a high atomization tip and i believe it is, whether i use wb or sb. if you were to take off the microscreen, the 2 series tip use them because the initial orifice or plate is smaller than the existing orifice. it creates a swirl topflow to the fluid stream. it works whether it is wb or sb. it is a concept of improving atomization. again, the microscreen the black tip seal with the i believe a 140 mesh screen does somewhat dimension the fluid flow along with the diffuser ie pre atomizer. personally i use aftermarket black microscreen to save a few dollars just an fyi. you may also have to jack up the fluid pressure to get the fluid flow you want again due to the smaller orific of the atomization plate and the microscreen i think you should have some success with it. i have.. good luck herb j

You aren’t using paper backed veneer are you? Is butyl acetate the recommended solvent for the product? Do you control the temp and humidity in your shop? Did it happen before summer heat and humidity started? How accurate is your catalyzing? I’ve seen too many finishers just pour it out with a guess on the amount.