Shadow Lines in Veneered Wood-Edged MDF Tops

Joints are telegraphing through veneer. Here are suggestions. April 18, 2010

Question
I build my own cores using MDF with a solid wood edge to allow me to create profiles on my tops. The problem I am having is that once my tops are sanded and clear coated, a shadow line appears where my MDF attaches to my natural wood. What is causing this problem, and how can I correct it?

Forum Responses
(Veneer Forum)
From contributor D:
This is called telegraphing and is caused by the substrate and solids moving at different rates, in response to changes in relative humidity.

Minimize the swings of humidity the tops are exposed to. There is only so much you can do here, but it will help. Crossbands on both faces (to preserve balance) will minimize the telegraphing. The thicker, the better, within reason. Another strategy is to add the solid wood flush to the veneer face, with a slight V-joint. Any difference will get lost in the V.

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From contributor V:
Let your solid edging dry more before you flush trim and veneer. Are you hot pressing? The heat from the press can dry out solid which has a higher humidity, causing it to shrink. It usually only goes one way, in that the solid shrinks, and that is it. This also happens with translucent veneers such as English sycamore, and sometimes maple. In that case you are seeing the darker MDF/particleboard through the veneer.

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From contributor J:
Try minimizing the solid on the top edge to about 1/8". You can rabbet out the bottom thicker there for your profile. The 1/8" generally doesn't telegraph and usually gets lost in a radiused edge.