How to avoid 'lensing'?

Question

I want to reduce the thickness of a small plastic washer. This is 12mm outside diameter (5mm ID) and 1.3mm thick and I want to reduce it to about 1.1 or 1.2mm thick manually using fine sandpaper. I wonder if I should worry about 'lensing'? that is, creating a concave surface?. Can anyone advise if I need worry about this and how to avoid it?

Answer 1

That's very perceptive because sandpaper may fill up with plastic residue unevenly and sand unevenly whether motion is back and forth or circular.  Perhaps you could place the washer in a firm-fitting hole of some material (wood, etc.) of equal thickness and sand the assembly from every angle.  The wood will keep the sandpaper evenly flat and a variety of angles of attack could avoid uneven sanding (especially with fine fresh sandpaper held by a sanding block).