Troubleshooting Finish Cracking over Veneer
Trying to determine the cause of finish defects over veneer — is the the core material, the adhesive, the veneer, or the finish? March 28, 2012
Question
A customer that I've built a lot of furniture for in the past ten years called me to tell me a lot of the finishes felt rough. When I examined the work (probably a dozen pieces) it seemed I was getting finish cracking over veneer checking: anything that was veneered felt almost furry, while the solid wood felt great.
All this work was finished with Campbell MagnaLac, which I'm very comfortable spraying and get good visual and tactile results. I'm wondering if catalyzed lacquers are too brittle for veneer for the long haul, and whether I should consider something more flexible, like a 2K poly? I'm doing a large corporate job now, lots of veneer, and I'm afraid that the CV I was going to spray might create the same cracking problems down the road. Does someone have advice on a flexible finish for both residential and commercial applications?
Forum Responses
(Finishing Forum)
From contributor J:
This sounds like a problem with the veneer rather than the finish. What sort of glue are you using?
From the original questioner:
Almost all those pieces are veneered using Unibond 800, a urea formaldehyde glue. There might be some that are pressed with aliphatic resin (white glue). Surprisingly, there is one that I could not press and I used contact cement and flexible veneer; there's no cracking or checking there (yet).
From contributor U:
Is the veneer bonded to a paper backing? If it is "paper backed" or "bubble free", the company usually has a disclaimer that suggests to use a non-catalized finish.
From the original questioner:
None of the veneer with the checking problem is paper backed; one of the few pieces that is not rough, in fact, is one which I could not press so is a paper-backed veneer bonded with contact cement. I know Oakwood Veneer suggests flexible finishes on veneer as you say, which is why I wonder whether a more flexible finish would have prevented this problem.
From the original questioner:
I have mostly stopped using Unibond as well, but principally because I had bleed through and staining issues on porous veneers, plus it's a mess, dangerous (formaldehyde), and expensive. I have been using white glue for veneers with good results. So you think that the veneer checking I'm getting is because of the Unibond? Any idea why that would happen?
From the original questioner:
Although I'm certainly capable of making many kinds of finishing mistakes, I don't think the problem I'm talking about is caused by too thick a coat of finish. I suspect that because the very first time I used pre-cat, maybe 15 years ago. I built up a beautiful finish which developed cracking throughout about six months later because I put on too much finish. When that happened I could see the hairline cracks throughout, but I couldn't really feel them. The cracking I'm describing in this thread feels like it's coming from the wood, like the wood itself has become rough or furry.
My typical pre-cat schedule is simply three coats of MagnaLac, thinned about 10%, sanding between coats with stearated paper. The final rub is determined by the sheen I'm trying to get.
From contributor W:
In my opinion the problem is more about the veneer then the coating. What kind of veneer did you use? There are some veneer such as crotch and burl that are easy to crack. Some hard wood such as rosewood is also easy to get crack problem. Did you check the moisture content of the veneer and core when you glued them? The difference of the moisture content between the core and veneer also could cause crack problem.
From the original questioner:
There are several different projects which suffered the cracking problem. The different veneers I used were "Karelian" birch burl, figured anigre, padauk, and purpleheart. I didn't check the moisture content of the veneers or the substrates. I traditionally have only monitored the M.C. of solid wood so this may well be a factor. What is odd, though, is that the pieces were built over a three year period, through humid summers and dry winters, with all materials having plenty of time to acclimatize to my shop.
When I read about veneer checking, the experts say all materials must be kept to a certain M.C., but in my small shop I don't have the capability to control humidity throughout the year. If M.C. is indeed the culprit, maybe I shouldn't use veneers at all.
From contributor O:
We have used (probably literally) a ton of Unibond for veneer work over the years, and sprayed a decent amount of Magnalac on it, too, though not in a long time. I wouldn't blame the Unibond - if it's used properly that is a great glue and extremely stable once cured. Bleed-through can be controlled with the use of the blocker that Vacuum Pressing Systems sells for the Unibond. I am guessing that you are vacuum pressing - a great system, but can accentuate bleed. I'm surprised you're not having bleed problems with PVA, unless you're using Titebond Cold Press for Veneer (with which we could never get satisfactory results on large pieces).
Magnalac: I used it because it was easy to get and easy to spray. If you aren't looking for too much out of it, it is very forgiving (I have sprayed in a barn during a snowstorm with an open window for intake air). But the build is very low, especially if you are thinning it.
Putting on my Forensic Woodworker hat, I'm wondering about your core stock. With the low film thickness of the Magnalac, is it possible that you are getting a lot of seasonal movement in your core stock? That could be pushing and pulling the veneer and Magnalac layers on top of it, stretching and crunching them and creating the rough surface. If it's an MDF or PB core it would be moving in all directions and so could do a number on the longwoods as well as the burl. If veneer core, did you cross the grain of the core with the longwood show veneer or go with it? The latter can sometimes pull the longwood grain apart - especially if you use a Baltic birch-type core, with that really thick outside layer that tends to move a lot.
Last thought: glue spread. Do you have any sense of how heavy a film of Unibond you used? The mention of bleed raises the question for me. In our experience, the right spread for Unibond is just a bit more than not enough (can be tricky to learn); if you put on a really heavy spread, I wonder if you have a thick slab of cured glue that is fragmenting with the seasonal movement. That might push up under the veneer and create a rough top surface.
From the original questioner:
My sense is that you're correct when you say that the Unibond is not the source of my roughness problem. I do use a vacuum veneering system. The bleed-through situation I had is a separate issue (and unfortunately was before VPS starting offering the blocker; when PVA bleeds through it doesn't stain light woods. I have a bedroom full of anigre veneer furniture that has splotches of yellowish stains from the Unibond.
I don't think I have an excessively heavy spread of glue, though obviously in some places (like the anigre bedroom set) I've used too much. I think the problem may be, as you say, some movement of the substrate that is creating problems in the veneer, which is transferring through the finish. Even if I were to use some thicker finish film, it would only slow the moisture transfer, not stop it; wouldn't I eventually still have this problem? It makes me think that I should have crossbanded over the MDF substrates, then laid my finish veneers. Would I have been better off using paper-backed veneers, which essentially come with their own crossband? I appreciate the help. Still not sure what I can do at this point to correct the problem.
