Hmm just to confirm, are you trying to add material to BSP geometry ?
If so, usually we used BSP for test play collision or block area, BSP from my understanding are very low polys and that could be why it affecting your material (especially if you use tessellation).
The solution would be to subdivide them or create normal static mesh from blender.
If that not the case, post some pics here of the artefacts and the workflow of your material.