I try to apply a surface treatmentbuilding object that represents finishing of a part
The term surface treatment is used in concrete detailing. For concrete parts surface treatment includes surface mixes, tiles, and flat finishes, such as troweling and sandblasting.
The term surface finish is used in steel detailing. For steel parts surface finishes include fire-proofing and unpainted areas.
to concrete spandrels, but why it is not created to one face of some spandrels?
The reason might be overlapping part cuts. If you change the part cuts not to overlap each other, then the surface treatment into part face will work fine.