I build lots of these barn doors for my customers and lately the larger doors have been cupping on both ends of the door. I use build a frame by using pocket holes and then glue and nail the tongue and groove boards to the back of the frame. I put glue in the grooves as well as where the boards meet the frame. I have been told that I should not be gluing the groove to allow the wood to expand and I am aware of that but I guess my question is, is that whats causing the doors to cup? Also, the doors are pre finished weather it be stained or painted. If the wood expands, then wouldn't the bare wood of the tongue be exposed and need to be painted and or stained? I don't see a good way to build these doors and not have either one of these issues.
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