How do I use a random orbital sander properly?
Working through the grits, letting the tool float, and why swirl marks come from pressure rather than the paper.
- Difficulty
- beginner
- Time
- 40 min
- Read
- 3 min
- Safety
- caution
Short answer
Let the sander's own weight do the work and keep it moving. Work up through the grits without skipping more than one step, and never tilt it onto an edge. Swirl marks are caused by pressure and by skipping grits.
A random orbital sander produces a flawless finish or a swirled mess depending almost entirely on how hard you push it.
Safety
Step by step
- Start at the right grit.80 for stripping and levelling, 120 for general preparation, 180 to 240 for finishing before paint or varnish. Starting too fine wastes time; starting too coarse leaves scratches you have to remove.
- Do not skip more than one grit.80 to 120 to 180 works. 80 straight to 180 leaves the coarse scratches in place, and they only appear once the finish goes on.
- Let it float.The tool's weight is roughly the right pressure. Pressing down stops the random orbit, which is exactly what creates swirl marks.
- Keep it moving.Slow overlapping passes, about 25mm per second. Holding it in one place digs a hollow that is very hard to see until the finish is applied.
- Start and stop on the surface.Switch on with the pad already flat on the work, and lift only after switching off. A spinning pad landing on the work gouges it.
- Keep the pad flat.Tilting onto an edge cuts a groove instantly. Sand edges with a block or by hand.
- Use the extraction.Connect a vacuum or use the dust bag. Dust between pad and work causes scratching, and MDF and old paint dust are genuinely harmful.
- Change paper when it stops cutting.Clogged paper burnishes rather than cuts and generates heat. It is cheap — change it far more often than feels necessary.
- Check with raking light.A lamp held low across the surface shows scratches and hollows that are invisible from above.
- Finish along the grain by hand.A final light pass with the grain using a sanding block removes any remaining swirls, particularly important before stain or oil.
Tips
- Let the sander float. Pressing down is what causes swirl marks.
- Never skip more than one grit — the coarse scratches only show once finish goes on.
- Check with a light held low across the surface before you finish.
Common mistakes
- Pressing down to sand faster — It stops the random orbital motion and turns the tool into a rotary sander, which cuts visible swirl marks into the surface.
- Jumping straight from coarse to fine grit — Fine paper cannot remove coarse scratches, it only polishes around them. They appear the moment stain or varnish goes on.
If it doesn't work
Swirl marks in the finish
Cause: Too much pressure, or grits skipped — Fix: Go back a grit, sand with light pressure, and work up in steps. Finish by hand along the grain.
Hollow or dip in the surface
Cause: Held in one place — Fix: Keep moving. Level the surrounding area rather than trying to sand the dip out.
Paper clogging immediately
Cause: Paint or resin loading, or sanding too soon after finishing — Fix: Use stearated paper for paint, and let finishes cure fully before sanding.
Gouge along one edge
Cause: Pad tilted — Fix: Keep flat, and sand edges by hand with a block.
Questions people ask
What grits do I actually need?
For most work, 80, 120 and 180. Add 240 if you are finishing with oil or a clear varnish where the surface shows.
Why does my sanded wood look fine until I stain it?
Coarse scratches remain that were invisible on bare timber. Stain settles into them and makes them obvious. Work up through the grits properly.
Can I use it on paint?
Yes, but check for lead in anything painted before 1992, and use extraction. Stearated paper resists clogging on paint.