How to Keep a Surface Flat While Hand-Sanding

A sanding block only carries the abrasive over the shape it is given. Find the highs, sand only those, and check against a straightedge so you do not dish the surface.

What you need

Model 45395Rubber Sanding Block $6.42 each$4.37 each at 100 or more

abrasive suited to the material and the next finish
straightedge or reference to check against
raking work light
cleaning cloth or vacuum

Steps

Decide what flat means here: flush with the wall around a repair, true to a mating part, or flat enough for the next finish.
Make sure the filler or coating is fully cured per its maker. Do not sand material that smears, pills, tears or is damp.
Clean the surface. Lay the straightedge across sound material beyond the work.
Find the highs where the straightedge touches first or the raking light casts a shadow. Mark only confirmed highs.
Cut a quarter sheet of abrasive and attach it to the 45395 flat and tight.
Sand the highs only, with light pressure and long strokes that run past the edges of the repair.
Recheck with the straightedge and light after each round. Stop when the high is gone. Sanding cannot raise a low.
Remove all residue. Check that the scratches are fine enough for the next coat.

Tips

A rigid block bridges hollows and cuts highs. Foam or a fingertip follows the shape and dishes the area.
Beware a high beside a low. Sanding both together removes the high and makes the low deeper.
A surface can be flat and still be too coarsely scratched for the next coat.
Check from more than one light direction. One angle hides defects.

Watch out

Paint in pre-1978 housing may contain lead. Sanding it falls under the EPA RRP rule for paid work.
Stop if you do not know what layer you are sanding. A change in color, hardness or dust is a sign.
Oil, wax, silicone or adhesive residue gets driven into scratches by sanding. Remove it first.

Questions about the right tool for your job? Email cs@hydestore.com or call 508-916-6442.