Q: We laminate a fine hardwood veneer (oak, cherry, maple, or hickory) onto an MDF core. We have an occasional piece that has a dark gray splotch or splotchy area on it. Is it the glue, the finish, or the wood?
A: I examined the pieces you sent and there are two clues. First, I notice that the dark areas, when you look closely, are associated with the grain patterns in the wood, especially cross grain. It is like end grain on the face of the veneer. This means that the wood can easily transport fluids or chemicals from the glue line to the surface in these areas. Second, a test showed that the stains were high in iron. So, I conclude that the stain is iron stain, where the tannic acid and iron interact to form iron tannate, a blue-black chemical. The iron must be in the adhesive and in certain areas of the wood. It is transported through the veneer to the surface.
Where is the iron coming from? It is my first guess that you are introducing the iron into the glue mixture through the water supply from a rusty tank, rusty pipe, or some other source. Other sources of iron can be steel wool that is not cleaned off the surface, or iron contaminated water or equipment in a water-based coating system.
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