Why can film sanding discs still leave deep scratches or random swirl marks?
Even with film-backed sanding discs, obvious deep scratches, random swirl marks, or sanding lines that are hard to remove later usually mean the sanding sequence or operating method still needs adjustment.
Common causes:
• Scratches from the previous grit step were not fully removed before moving to a finer grade.
• The backup pad is too hard or the contact surface is uneven, creating excessive local pressure.
• The disc is already dull or loaded, but sanding continues with heavy force.
• Curves and edges are sanded without an interface pad or soft backing, which makes random deep marks more likely.
How to improve the finish:
• Make sure each grit step has removed the previous scratch pattern evenly before moving on.
• Match pad hardness to the shape of the workpiece, and use an interface pad on complex curves whenever possible.
• Let the disc cut instead of forcing it with pressure, and replace it once heat builds up or cutting slows down.
• Keep the surface clean before and during sanding so coarse residue is not dragged back across the finish.
