Problem with veneer over MDF

Veneer applied over MDF is splitting and cracking - what could be the cause? March 21, 2002

We outsourced two jobs that required veneer over MDF - one for cabinet doors, the other for interior doors. In both instances, within about 6 weeks following installation, the veneer split, cracked and in some cases delaminated. My first thought is that moisture caused the problem but now everybody including the architect is baffled.

The doors were delivered to the job(s) directly from the manufacturers and only after, the air conditioning was operational. On delivery and installation they appeared to be well made and had no sign of the problems now evident. Of course, the manufacturers both claim their products and manufacturing methods are without cause.

Any ideas on the cause?

Forum Responses
What location are these jobs in and where were the doors built? What time of year was all this done? Were the veneers pressed on the doors? What type of adhesive was used? What type of finish was put on the doors? How long after the doors were built was the finish applied?

Locke Wilde, forum technical advisor

From the original questioner:
The job is in Montgomery, AL and currently ongoing. Doors where originally ordered from the manufacturer in October and arrived at the job in December and January. There is a lacquer finish applied by the manufacturer. I do not know about the type of adhesive used or whether the veneer was "pressed".

We make cabinet doors for sale to cabinetmakers. In the past we have used only solid hardwoods. Recently we were asked to make a large number of cabinet doors using MDF. If this problem with MDF veneer is a general thing, we will decline the order.

MDF is an ideal substrate and in fact the preferred substrate by many when laying up hardwood veneer. The fine grain, smooth surface on the MDF allows intimate contact and no telegraphing of defects or rough surface blemishes. Locke will bring the issue to a realistic conclusion and that conclusion won't be the MDF as the substrate. My remark assumes that the MDF is within a proper moisture content range and without a manufacturing defect.

Are you speaking of 1-3/8' X 3' X 7' doors?

MDF is the substrate of choice by most large manufacturers because of consistency. As with any substrate, the one factor that can be the most damaging is moisture. Check the moisture content of the substrate on one of the blistered doors. Use a moisture meter that has prongs that penetrate into the surface, going past the veneer face, reading the moisture in the MDF board. The moisture in the MDF should be 7% to 9%.

You say you have checks in the face, correct? These are splits going with the grain in the soft grain? You say there is delamination - this means blisters or veneer coming off the substrate, correct?

These two problems are caused by different things. The delamination is a failure of adhesion due to one or more things. One would be not enough pressure applied for the adhesive to make a proper bond. Another would be not enough adhesive used for two substrates to make a bond. And another would be that moisture, which shouldn't have been there, was present in one or both of the substrates.

As for the checks, these could be caused by a few problems. One - moisture was trapped under the seal coat. Two - the wrong seal coat was used. Three - the finish was rushed and not allowed to flash off between coats. Four - more of the product was applied at one time than the product was designed for, in wet mil thickness.

Wood moves anytime moisture is added to or taken away from. If the veneer is not bonded properly to the substrate when the finish is applied and the wood veneer expands and contracts, blisters, checks and splits will appear.

There are a couple of questions I would ask the door manufacturer. What is your manufacturing process? Do you make your own veneer faces from raw veneer? Do you use sheet veneer on the face? If yes, what type of backer and what type of adhesive? Do you use veneer faces? If yes, what type of adhesive? What type of pressure is used to apply the veneer to the substrate?

Locke Wilde, forum technical advisor

From the original questioner:
I have checked the moisture content with a 2 prong meter and found the average moisture content of the MDF to be 9%. This leads me to believe one of three things:
1) a more-than-desirable moisture content was evident in the MDF prior to the veneer being applied, and was not acclimated prior to the finish.
2) the finish was rushed and a heavier application was applied.
3) adequate drying time for the finish was not permitted between coats.

The doors I refer to are 1-3/8' X 3' X 7' doors. The manufacturer is being very careful as to what he says about the process and conformance to the process. Of course, their standard specifications are available. The questions are a) did the manufacturer follow his own established guidelines or b) is it a moisture problem created after delivery to the job. In this instance, and since the doors were purchased by the contractor directly from us, we are unsure.

The doors were delivered following written assurances from the contractor that the job conditions "met with the specification requirements" for delivery of the doors and temperature and moisture conditions "are controlled". Initially I refused to arrange delivery without these assurances for fear of a similar problem.

Is it possible that the adhesive used had higher than normal shrinkage rates as it dried? If so, it seems that this would cause much greater stress to the veneer pulling it, resulting in cracks, checks or whatever. Is this a reasonable cause?

It might be possible that an adhesive had a higher water % in one batch than another, and that could make a difference.

Locke Wilde, forum technical advisor

From the original questioner:
Here is what appears to be the cause. Prior to delivery to the project, moisture content was verified and found to be in compliance, and certified by the architect. The contractor certified that job conditions were in compliance with the specifications, moisture was under control, etc. It turns out that the contractor's certification was not true and moisture readings were excessive, caused by incomplete roofing, windows and no control.

Fortunately enough, the contractor signed acceptance of the doors and in doing so, certified once again that job conditions are in compliance with the specifications. (We have that language in our delivery receipts.)

It is now just a waiting game - the owner and architect have taken our side and place blame on the contractor, who said he was just trying to get back on schedule. I hope he learned a lesson…

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

Comment from contributor A:
I have used MDF (medium density fibreboard) for many years and have found that when laminating it is best to put on 2 coats of the adhesive due to the fact that the first coat will be absorbed, the second coat is the bonding coat. If this is not done then delamination will occur. You should also damp the reverse side of all veneer prior to gluing.

Comment from contributor B:

I agree, the best way I have found to do MDF substrate is to put a thin coat of adhesive. It acts as a sealer, and allows the second coat to do its job. Also, a press is the way to go with these doors. Not sure if it was mentioned in their manufacturing process.

Comment from contributor C:
Cracking the veneer - it is possible that veneer was not dry enough and due to higher temperatures in a press later on it cracked.

- MDF surface might not be flat enough, sanding calibration is required very often.
- Less amount of glue.
- Very short open time of the glue so that it can not be spread fast enough.
- Very short reactivity time, meaning boards are not put into a warm press fast enough and press was not closed fast enough.
- Thickness tolerance of the veneer is not proper. If thickness tolerance is too large, veneer will delaminate after staining. This can not be seen before, i.e. 0,6 mm can vary from 0,57 - 0,64 mm but if nominal thickness was bought as 0,6 mm but supplied from 0,50 - 0,70 you are in trouble.

Comment from contributor D:
If sheet veneer was used: Is the delam between the wood veneer and the paper or fleece? Or is the delam between the paper or fleece and the MDF? When applying a backer to the sheet veneer, there is a processing period in the press. If this is rushed there can be a delam between the veneer and the paper or fleece. This will not show up until applied and in the field. Heat will cause this problem. Glue combination will cause this problem also. PVA and EVA glues have different melting points and could cause some of the delam problems. Heat is a big factor and time in curing.

Comment from contributor E:
I had this same thing happen using quarter cut fir on MDF core with waterbase contact cement. The lacquer used was a two part. We figured that the lacquer was put on too heavy. When it dries it shrinks. As it shrinks, it's going to either crack (if the veneer was glued on stronger) or lift the veneer, which it did.

Comment from contributor F:
It could very well be that control of moisture at the job site was the source of your problem. It is very typical for contractors to be lackadaisical about the building environment. The only thing is that using MDF exacerbates the problem. It is a very poor material in terms of stability, even though it seems to be sold as a very stable product. I have seen MDF shrink a lot.

I worked in a shop in the nineties that thought it was a god send and we used it a lot for veneer cores as well as for paint surfaces. I don't touch it anymore if I can. It is heavy, its dust is a horrible nuisance, it swells irreversibly with water contact (and it is used for baseboard), it inevitably shrinks over time, it is toxic, and requires a great deal of energy to produce. I would suggest using 1/8 inch masonite for solid core door skins under veneer and particle board or plywood for cabinet door cores.