This question already has an answer here:
I have imported a mesh (AutoCad DXF) and I'm experiencing strange shading behavior. I've done the following to try and address the problem with no luck:
Delete Loose Geometry
Cleared Seam, Sharp, and Bevel Weights
There are no modifiers on the mesh.
The original imported mesh looks fine in smooth shading but I need the mesh to be in quads which causes the shading to mess up as shown in photo. I can add my .blend file but I'm not sure how to do that.