Paper rounding over a crisp edge

Sandpaper rounds a crisp edge when it flexes over the corner or your sanding strokes travel across it. To keep the edge sharp, support the paper with a flat, firm block, sand only the face you mean to smooth, and stop your strokes just short of the corner.

Why paper rounds edges {#why-paper-rounds-edges}

Loose sandpaper bends easily under hand pressure. As it wraps around an edge, the abrasive touches both faces and gradually removes the corner. A soft sanding sponge or fingertip pressure can make this more likely because the backing gives way around the profile.

Repeated strokes that cross the corner have the same effect, even with a sanding block. The finer the finish work, the easier it is to unintentionally soften an edge through repeated passes.

How to keep an edge crisp {#keep-a-crisp-edge}

1. Use a firm, flat backing. Wrap the paper around a hard sanding block, or use a block designed for flat surfaces. Avoid a sponge when the corner must stay sharp.
2. Keep the block flat on the face. Use light, even pressure. Don’t tilt the block so its edge rides over the corner.
3. Stop each stroke before the edge. Sand the face up to the corner without pulling the paper across it. Turn the workpiece to sand the adjoining face separately, if needed.
4. Use only as many passes as necessary. Check the edge often, especially during fine sanding or between finish coats. Follow the coating maker’s instructions for grit and sanding between coats.
5. Protect the edge if appropriate. For a painted or finished surface, masking the corner can help keep sanding confined to the face. Remove tape carefully and follow the finish manufacturer’s guidance.

If you actually want to ease the corner, do it deliberately with a few light, consistent passes rather than letting flexible paper round it unevenly.

Fixing an edge that has already rounded {#fix-a-rounded-edge}

If the rounding is slight, a flat block may help restore a sharper line. Hold the block square to one face and sand carefully, then check the adjoining face. Avoid trying to correct a deep or uneven round by aggressively sanding one side; that can flatten the face or change the part’s dimensions.

On thin veneer, a finished surface, or a piece where dimensions matter, stop before sanding through or removing too much material. A deeper rounded edge may require refinishing or replacing the workpiece rather than more sanding.

Choose paper and backing for the job {#choose-paper-and-backing}

Use a grit suitable for the material and the amount of material you need to remove; finer paper is generally suited to light smoothing, not correcting a large defect. A rigid block supports sheet paper for flat faces, while a soft sponge is better reserved for contours where some flexibility is useful. Check the abrasive and finish instructions when working on coated surfaces.