Q. What causes small little bumps (we might even call them pimples) that we can see in our gloss finish of veneer wrapped over MDF? They are hard to feel with our hand, but are real obvious by eye. When we examine the pimple, we find that the veneer seems really thin at that spot and the MDF core is right below the surface. (The note included more info on laminating and finishing.) We have done this veneering and finishing for quite some time and only now are we seeing the problem.
A. What is happening is that the MDF core material has some compressed wood at the surface. This compressed wood will, especially if there is any moisture present, “uncompress” or develop into a pimple on the surface, over time. As you state, it is hard to see or feel these in the final product. With the MDF, before the veneer is applied, the bumps would be really hard to detect.
After applying the veneer to the MDF core with pimples and then sanding the veneered panel to get the final smooth flat surface, everything will appear OK. However, wherever there is a pimple in the MDF, the veneer will be raised. Now when sanding the panel prior to finishing, there would have been a lot of the veneer sanded off right at the pimple (that is, the veneer would now be thin) and the MDF would be close to the surface. Now, when we use a waterborne stain, the moisture that goes into the MDF in these “thin” spots causes the MDF to swell a little bit in these localized spots. This swelling will show up as a bump or pimple in the finish.
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The cure is in the manufacturing of the MDF. Although the waterborne finish, which is so common today and must be considered by all manufacturers, is part of the scenario, to blame the WB finish is like blaming the hard ground for causing the sudden stop when you jump out the fifth floor window. Without the waterborne finish, the bumps might not have been as dramatic and would not have showed up promptly after finishing, but would eventually show up later in the customer’s home or office.
Incidentally, we see the same pimples when using a solid wood core with a veneer lamination and even with solid wood itself. In this case, the wood was planed smooth using an inadequate dust system. The chips hung on the knives and compressed localized areas when they rubbed on the surface of the wood. Although the surface can be sanded smooth, the compressed areas were too deep to be sanded off and so they will pop up later, especially with moisture.
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