Q: We are noticing some marks in the veneer that we glue to an MDF core. The marks look like parallel lines, very close together in certain parts of the panel, and appear only after the panel is completed and has "aged" for several weeks. Help!
A: I suspect that we will find that the stain is iron related. Do the lines agree with the ridges in the spreader you use? Do they run the same direction? Perhaps the adhesive is mixed with some iron containing water?
Perhaps you could have your adhesive supplier test the mixed glue for iron. To prove it is the adhesive, could you spread some panels with the spreading marks at 90 degrees to normal?
Perhaps also you could try sanding and any other appropriate operation at 90 degrees to see if the marks also rotate. Also try rotating the substrate, but keep the glue spreading on the veneer the same.
You might also consider a film adhesive as a test to see if the problem stops. Although you sent data on finishing, I am not an expert in this area. But the process you use sounds perfect for a quality finishing job.
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