Bubbles in veneer

Why are bubbles appearing in veneer applied with PVA glue in a bag press? March 21, 2002

We have been using Multibond 2015, which is a type II PVA adhesive, to glue down our solid wood (non-backed) veneers for the last 5 years. During that time we have been finishing with General Finish poly/acrylic blend, which is a water-based lacquer. Substrate is MDF.

Recently, we have experienced problems with bubbles in the veneers after spraying on the finish. We have had the most problems with pommelle sapele, but also birdseye maple. What is the best way to prevent the water in the lacquer from stressing the veneer? Should we switch to urea adhesives? We have used Multibond because it is easy to apply and does not need mixing.

Forum Responses
From contributor T:
You could size and flatten the veneer with a veneer softening solution: 3 parts water, 2 parts white or yellow glue, 1 part pure grain alcohol, 1 part Glycerine. Press between 1 layer of fiberglass window screen and 3 - 5 layers of newspaper. I use a vacuum bag but have used building blocks to press the veneer. Change the newspaper 3 - 5 times the first day. The second day you can press the veneer and paper together for another 3 - 5 changes or until you think it's dry. When you press your veneer the sizing should solve your problem. Have you tried urea resin glue? I found out the hard way that water-based glue and veneer don't mix unless the veneer is sized.

From the original questioner:
We had been flattening the veneer with GF-20 veneer softener from Veneer Systems. I am concerned that using a flattener which has white glue in it would inhibit staining - have you had any experience with staining problems after sizing?

From contributor R:
What type of press are you using for layups? I currently use a vacuum press and have found PVA to be an unacceptable choice for glue. Urea is the only way to go.

Urea is great (best), but usually in a hot press. PVA has worked okay in a cold pressing application. PVA should be pretty good with a bag press. I'm curious to learn the down side of PVA that you've experienced.

From contributor T:
If you use a sharp wood scraper and sand the veneer after pressing, there should be no problem staining. After scraping and sanding, go over your work with denatured or grain alcohol. You will notice any glue residue.

The downside of PVA glue is when I start my project I use dry veneer. When I apply PVA on an unsized veneer, the veneer cups and splits due to the water content of the glue. The second reason I don't like to use PVA on my work is that most of the projects I make are under tension and/or a compound curve. Add any amount of heat and the PVA turns soft and lets go. Urea, when catalyzed and warm, shatters. Dried PVA, when warm, looks and feels like old bubble gum. That's why musical instrument makers advise us never to transport stringed instruments in the trunk of a car. You end up with exotic kindling.

From contributor R:
I tried for the longest time to get PVA to work in my bag press. Then it dawned on me that putting glue that cures by evaporation in a sealed bag with no air is like putting it back in the bottle. To complicate that further, I knew that sometimes PVA was successful, but why?

Then I noticed that on some door panels I would get either spotty delamination (starved areas) or wrinkles (too heavy), even though I was meticulous when applying an even glue line. I figure the glue was starting to flash before I actually got it into the bag under full pressure, so I switched to a PVA glue formulated for veneer. This glue appeared to work fine at first, then after a few days my perfect seams would pull and reveal an unsightly glue line.

My take on this is that the moisture in the glue has nowhere to go in a vacuum bag, so it diffuses into the wood cells that no longer have air in them, making the veneer take on too much excess moisture. That, combined with the creep at the seams problem, makes this glue unsuitable.

Why fight it? Powdered urea or 2 part catalyzed have none of these problems. Using a glue that cures by a chemical action and leaves a rigid glue line solves these problems in a vacuum.

Now if I could just solve my glue bleed on open porous veneers. Anyone tried a shot of vynal sealer on the glue side of raw veneer just before bagging?

From the original questioner:
We are using a bag press. Multibond 2015 made by Franklin is a PVA that is specially formulated for cold press veneering. It contains flour to prevent bleed-through in open-pored woods, and it works pretty well.

I think that contributor T's approach to the problem sounds way too labor intensive for a production shop. Contributor R's theory that the glue dries by evaporation - I am not sure that is correct, either, although a glue chemist would be a better authority. I understood that PVA adhesives bond at least partially by hydrogen bonds between the water molecules in the glue and hydrogen atoms in the organic matter.

We have been using our vacuum press with PVA glues for 11 years, gluing something almost every day, and have not had consistently bad results. I would say that we experience above a 95% success rate. Generally, when we do experience bubbling, it is at most 1 or 2 spots in a tabletop, i.e. much less than 1% of the total bonded area. I think that there are two possibilities: water is working its way into the glue line from the finish by way of the short grain in heavily figured veneers and weakening it, and the same moisture is causing the veneer to want to expand. The question: does urea formaldehyde make a satisfactory substitute for PVA in terms of ease of use and press cycle time (we are already leaving the panels in overnight).

Stay with the PVA in your bag press. Urea really works better with heat (190 degrees). If you would add a little heat to your bag you can cut that lay-up time by a lot. Just wrap your bag in an electric blanket and turn it on medium-high for about 90 minutes.

Locke Wilde, forum technical advisor

From contributor T:
The urea I use, Unibond 800, has a low range of 55 degrees. I control my vac bag at 85-90 degrees and have a 3 hour set.

Contributor R, regarding bleed-through on open pore species, you should be sizing these veneers first. We have a Bordens product called glue size. It is a resin emulsin that acts like a "sanding sealer". It is applied on the back of the veneer to seal pores - thus reducing bleed through.

As an added bonus, it provides a uniform density to the veneer face. This allows for a more uniform finish and tone because the finishes are not drawn into open areas, and topcoating denser areas. It comes full strength - use full strength for really open pore (red oak and ash) and dilute with water 50/50 to cya yourself on other veneers.

These are the reasons for bubbles:
Substrate: MDF can have a slightly oily layer that repels glue. Try particleboard.
Pressure: Not enough.
Drying time: Veneer should not feel damp when you take it out of the press.
Glue: Too much is bad, too little is worse. You need to see a slight squeeze out at the edges.

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

Comment from contributor A:
We have six bags that run continuously five days a week. For most species we stick with a casein PVA and have no problems. For exotics such as rosewood, ebony, etc. or problem species such as maple, pommele, or anything that reacts quickly to water, we use Unibond 800 and have no problems. Curved work is extremely stable with the Unibond also and you can add an aniline dye to tint the color of the glue to help combat gluelines showing at seams. The key to any press operation is to keep a log of species and enviromental conditions and remedies. Above all, when using a bag press, getting your work under pressure quickly is the key. With bigger or oversized bags, try using a shop vac to assist in speeding up the vacumm process. You'll be pleasantly surprised by the results.

Comment from contributor F:
The problems you're facing with bubbles in the veneer can be attributed to a number of reasons. Let's talk about your substrate first. Are you calibrating your core prior to press? Although various forms of substrate appear to be flat, almost always there will be a thickness difference. Even shallow spots of .015" can cause de-lam between your veneer and substrate. Calibrate if possible - you'll get a better glue bond and a better finished surface.

Next, let's talk about your glue. I have found that using a PVA glue in a vacuum is useless, because PVAs cure by an evaporation process. Believe it or not, most PVAs are up to 74% water, and only 26% solids. It takes air for this process to happen, and in a vacuum there is no air. Therefore, the glue cannot fully cure while in the vacuum. When you remove the work from the bag, it initially appears to me bonded, but because it is not and you have removed pressure from the piece, the glue will cure, but it will not have anything to help it bond the two surfaces together. Now on the other hand, urea-formaldehyde glue cures by a chemical process, no air needed, therefore it is more ideal for a vacuum bag application. Ever notice how urea even cures underwater? I would try a good urea glue for your vacuum process. Borden makes a good product called Cascamite. Sometimes you have to use steps that take a little longer, but in the long run you'll save time.