A backing plate is the connection between the machine and the pad or sanding disc. The diameter, stiffness, and quality of the backing plate partly determine how evenly the pad contacts the paint and how pressure is distributed. A poor or worn backing plate gives uneven results even if the pad itself is fine.

Backing plates are sold by diameter (typically 75, 80, 125, or 150 mm) and by attachment type (M14 thread or 5/16 UNC for European and American machines respectively). Always check that the diameter matches your pad and that the thread fits your machine.