Sandpaper Makes It Flat, a Sanding Sponge Makes It Smooth

The Problem

Sanding sponges feel nicer to use than sandpaper, so plenty of DIYers reach for the sponge every time — and then wonder why their patched wall still shows a bump under the paint. The two tools do different jobs, and swapping one for the other changes what your sanding accomplishes.

The Technique

The distinction, from a painter who has this argument with every new hire:

• Sandpaper makes things flat. Backed by a flat block, paper shears high spots off — a patch ridge, a run, a proud spot gets cut down level with the surrounding surface.
• A sanding sponge makes things smooth. The sponge flexes and follows the surface, polishing whatever shape is already there. Sand a bump with a sponge and you get a smooth bump — still visible under paint, just silkier.

So the schedule for paint prep is:

1. Sandpaper for every working step: before primer goes on, after patches, after the primer coat, after the first finish coat. These are the passes that need to remove imperfections.
2. The sponge exactly once: immediately before the final coat. By then the surface is already flat — the sponge's job is a final gentle smoothing that won't cut through what's underneath.