Contact Cement Failure with Veneer after Finishing
Is the problem caused by the finish, the solvent carrier, the adhesive, or what? September 25, 2014
Question (WOODWEB Member) :
I'm hoping someone has a suggestion. We veneered some curved panels with wood-on-wood backed veneer using flammable contact adhesive. Our finish coats seem to be soaking through the wood-backed veneer and dissolving the contact cement, causing bubbles. We're finishing with pre-catalyzed clear sealer and top coat. The sealer didn't seem to be a problem, but the top coat is. Can anyone suggest something we might spray on the surface to create a barrier? I was thinking shellac or a water based sealer. Any suggestions would be appreciated.
From contributor X:
The real problem is using contact cement for veneer work. I would say use a better glue but if you already did and just need to figure out how to work around it. Then I would probably try a waterbased system so it won't attack the glue.
From Contributor S
Contact cement or spray adhesive is a weak glue to use especially with a solvent based finish. As solvent based finishes cure they generate heat and shrink. The heat is softening the glue and as the coatings shrink it is pulling the veneer off. Get a better glue or use water-based finish. I would suggest getting a better type of glue as I have seen the heat from sun lit rooms pull veneer off over time.
From contributor N:
A friend of mine uses a lot of veneer in his shop, he does not like to use contact cement because of problems lifting while finishing and he says when he does use contact. He waits a couple days before staining and finishing and it seems to solve the problem for him.
From contributor F:
Echoing others responses, contact adhesive is for laminate, not veneer. It is quite near impossible for the finish to do what you are describing unless you have sanded the veneer down to nothing and you are applying the finish with a fire hose. To demonstrate this take a piece of the wood on wood veneer and pour some of your finish on it, let it sit for as long as you like then turn it over to see if any finish is soaking through the veneer. Even after overnight, the bottom of the veneer will be dry as a bone.
The problem with contact adhesive is that most people who use it with veneer will use too much adhesive, not let it flash long enough and worst of all will use a J roller to press it down with and a J roller simply can't produce enough pressure. This leads to a poor bond and therefore bubbling. The contact will also remain flexible and allows the veneer to move but the substrate and the veneer will not always expand and contract at the same rates with changes in temperature and humidity. So when one moves more than the other, something has to give and the veneer cracks. If you absolutely, positively must use contact for veneer, find a veneer scraper and learn how to use it properly. I remember reading an article some years back about this and the author said "if you aren't drenched in sweat when you’re done scraping down a piece of veneer, you didn't apply enough
pressure". Better yet use the proper glue.
From contributor Y:
Try 3M FastBond 30 Clear. It goes down like paint and has a long open time so you can smooth it out to a thin film. It's water soluble so it cleans up great and has no odor. Solvent doesn't attack it either. We use it successfully with paperback veneers and laminates.
From Gene Wengert, forum technical advisor:
Perhaps it is just playing with words, but it is not the finish but the solvent in or of the finish. The solvent cannot be seen as it moves through veneer, but the solvent vapors do indeed cause loss of adhesion. Note that not all solvents affect adhesion.