How to sand concave topographical surfaces without buying a fancy foam-padded attachment for your Random Orbital Sander
Step 1: Cut Polystyrene Into a Circle the Same Shape As Your Pad
Cut Polystyrene into a circle the same shape as your pad
Step 2: Spray Adhere One Circular Sheet of Hook&loop to Foam
use spray adhesive to attach hook&loop paper with the fuzzy side out.
Step 3: Attach Sanding Disk to the Other Side
attach sanding disk (sand paper out) to the other side of the foam.
now you have a thickened piece of sand paper backed by foam that can follow concave and convex surfaces