Troubleshooting Finish Cracking (Checking) on Veneer

      Here's an extended detailed discussion of finish cracks that follow wood veneer grain patterns. Is it wood movement, a veneer adhesion issue, a finish formula problem, or a finish application problem? March 13, 2009

Question
I installed a wall unit consisting of Macassar ebony boxes with mahogany doors three weeks ago - veneer over MDF. The finish is beginning to develop checks along the grain pattern. This is with the ebony only and not the mahogany. The finish consisted of a one coat of Bullseye Sealcoat,crystalac grain filler, and Sherwin Williams nitrocellulose lacquer. I have finished a lot of ebony before without the crystalac without any problems so I tend to suspect the crystalac which was applied heavier to the ebony. The checks appear slightly whitish as though the lacquer cracked? I realize ebony can be slightly oily and the piece is in a drier climate than my shop, but that is nothing new as I've finished ebony many times in the past with no problem. I'm afraid the real answer I'm after is what would be the least messy way to strip off the finish.

Forum Responses
(Finishing Forum)
From contributor J:
How was the veneer applied? The first thing I suspect when this happens is not the finish itself but the glue and pressing of the veneer. Contact adhesive and a J roller are a recipe for failure every time. Are the mahogany doors veneer as well, glued and pressed the same as the ebony?



From Gene Wengert, forum technical advisor:
Finishes do not crack unless the substrate underneath is moving. Today's finishes are very brittle (compared to spar varnish, for example), so it takes less movement of the substrate to crack the finish. When a complex finish cracks, the crack will typically appear white on the edges, reflecting the broken molecules.

If the substrate moved rather uniformly, then the finish would be stressed uniformly and probably not crack easily. The problem we have is when the finish has a pre-existing crack (not too uncommon in some veneer that is of medium or low quality - lower cost often). Now, when the veneer shrinks just a bit with the exposure to low interior wintertime humidities, rather than shrink all over, the shrinkage will be concentrated in the crack and then this concentrated stress cracks the finish.

The cure is to purchase only the best veneer (manufacturing techniques are the best, that is), minimize moisture changes after finishing, and use the most flexible finish possible. (I also think that a thicker finish (several coats) can absorb stress better, but have not seen others adopt or accept this concept. There seems to be a trend to thinner finishes, which makes cracking more likely).



From the original questioner:
First, thanks for the responses. Both the ebony and the mahogany were laid up on MDF by a commercial veneer company, not that that means there cannot be an adhesion problem. In the past I've never had a problem like this even on solid wood where there is sure to be some movement. That led me to suspect either the finish (Sherwin Williams pre cat T77C35 if that helps) was more brittle or reacted with the crystalac which I've never used before. Im praying that the veneer is stuck down good.


From contributor R:
Modern finishes can crack when applied past their maximum dry film thickness. They also can crack due to a cold climate below 65 degrees before they have had their 48 hour cure time and some finishes can crack if you use more than the suggested amount of catalyst. Some pre-cat lacquers have a max dry film thickness of two mils! Check with SW to see what the specs are for their product.


From contributor J:
If you see white lines in the cracks than this is most definitely the wood that is checking. When just the finish cracks thats all it does is crack - so all you see are cracks. When the wood checks or cracks and lifts you will see white lines indicating that you are now seeing the edge of the crack which denotes it was indeed lifted up by the wood that lifted up.

I had this exact problem three times on my 15 years - two times on veneered maple on columns and another time on plain sliced oak, contact cemented onto white melamine in the middle of winter, then brought into a hot Manhattan apartment when I was done. The cabinet maker actually blamed me for it and I took a $4,000 hit on it.



From contributor R:
Here is a picture of a table that I had to strip recently. I was able to speak to the finisher who was working when the job was done and know that CV was used as the finish. These are definitely checks due to exceeding dry film thickness. After stripping the veneer was fine and I was able to refinish the table with polyester sealer and a 2k topcoat with no problems.

As you can see in this picture and the next the checks are white around the edges. In my experience they will always be white whether its the veneer or the finish failing. The only way to tell is to take it back down to raw wood and use a good magnifier. On this table the checking was only in the topcoat layer, the sealer build coat was intact.


Click here for higher quality, full size image

Here is a close-up view. Anytime contact cement is used in veneer work I would be very concerned.


Click here for higher quality, full size image



From Gene Wengert, forum technical advisor:
To contributor R: Why would finish cracks follow the grain of the wood? This is what I seem to see in the photo above - the cracks are following vessel cells.


From contributor R:
It may depend on the wood species Gene. I have seen them where they shatter just like glass and also where they follow the grain. I believe they start fracturing with the grain structure and then join together to get the shattered look.

This table was veneered in a heated veneer press using urea formaldehyde glue. When the old finish was completely removed there were no checks in the veneer at all. I refinished it in a high gloss and it looked beautiful.



From Gene Wengert, forum technical advisor:
I wonder if it is related to the first finish. When we finish for the first time, if the MC is wrong, we have an elastic substrate (wood) that is going to move a bit with a rigid, brittle finish. If we let the substrate move before finishing, then all is ok. This is why refinishing works so well - we have gotten rid of the main culprit to failure and that is high substrate movement. I do not believe that we have to have checks in the veneer. However, from a microscopic view, wherever we have vessels, we have the potential for more movement.

In the picture you presented, the number of cracks is huge and if we total up all the small cracks going across the grain, we see that there has been a lot of movement (or stress development) overall. The source of such movement could certainly be the elastic substrate coated with a rigid finish and then the substrate moves. (Analogy: Think of coating a rubber band with an inflexible coating, letting the coating dry, and then stretching the band a little bit.)

Incidentally, I can show you a veneer or solid wood surface that has lots of little checks (seen when the RH is real low), but because the RH is not real low it is impossible, even with 15x magnification, to see them. They close up tightly. I have also seen veneer that had finish cracks only on one side (the loose side). This was seen when the veneer was reversed face-to-back, such as when book-matching. What do you think? One other comment. I have seen cases that looked like your photo and at each crack there was a slight raised area right at the crack. Have you seen this? Any idea on a cause?

It is my thought that the cause of such raising is that the substrate is expanding, while the rigid finish cannot, so eventually the finish fails and lifts up to accommodate the larger substrate. (If the substrate were shrinking the finish would develop gaps).



From contributor R:
I am sure that substrate movement can definitely be a factor as well as MC. I don't believe that to be the case in this example. The company I work for now is a custom veneer shop. I haven't seen a single case of veneer movement since I have been there. The only problem I have seen is with their stitching. I get slight gaps in some veneers between book matched leaves.

Modern catalyzed finishes are very brittle and prone to checking on their own even when applied to a non-wood substrate. I would get in touch with chemists from a couple of the large finish companies and they can give you a better insight as to how this occurs.

From my experience CV is the worst culprit (which includes many pre-cat lacquers which are just CV hybrids) and you can cause checking 3 different ways:

1. Cold checking: If the finish drops below 65 degrees anytime within 48 hours of spraying.

2. Over catalyzation: Some finishers like to double the catalyst because it makes the CV dry faster in dusty environments.

3. Exceeding the max dry film thickness: Many shops use Kremlin air assisted guns these days and they pump out a lot of material fast. I have seen guys apply over 12 mils wet in one pass using this equipment.

I hope this helps Gene. I am just a finisher not a chemist and have found it better to talk to the chemist rather than the salesman when it comes to issues such as these.



From contributor S:
Gene, the rise at a crack is called tenting. My experience with tenting has always been that the substrate has shrunk causing the veneer to raise slightly along the grain (the most vulnerable point for movement) as it has lost surface area to adhere to and it has to go somewhere.

Most coatings (there are exceptions) when cracked show up as white lines even when the veneer is not checked, as the light refraction has been broken by the break. Just like a crack in a piece of glass, the light cannot pass through and the eye sees this as (a white line in coatings) a dark line in glass. If the crack is repaired with an adhesive that has a similar light refraction index as the glass (or crystal) the break line virtually disappears as the light will pass through the glass and adhesive at the same rate.



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