Rotary vs orbital sander: which to choose and how to use it

The guidance below is workshop practice, labelled as such; the dust-control section is the exception, where every fact carries a reference to the Health and Safety Executive's wood dust guidance and resolves to the ledger at the foot of the page. Where this article and your machine's manual disagree, the manual wins.

§1 The choiceRotary vs orbital sander

The two machines are often confused because both drive a round abrasive disc, but they move it differently, and that difference decides what each is for.

A rotary sander — a disc sander, an abrasive disc on an angle grinder, a rotary polisher fitted with paper — spins the pad about a fixed centre. Every grain travels the same circle on every revolution, fast at the rim and barely moving at the centre. That is why it removes stock quickly, and why it leaves a tell-tale pattern: a set of arcs, all curving the same way, that run across the grain and show the moment a stain or clear finish goes on. A hand-held rotary disc is normally worked at a slight angle on one part of its face, because a spinning disc laid flat grabs and skates; that same angle concentrates the cut and the heat on a small patch, so a rotary digs, dishes and scorches wherever it pauses.

An orbital sander — the quarter-sheet finishing sander is the plain form — moves the whole pad in a small, tight orbit. The pad circles; it does not spin. Every grain scratches a tiny loop, so the scratches overlap in every direction and no long cross-grain arc is left behind. The random orbital sander most workshops own adds a second motion: the pad is free to turn on a bearing while it is driven round that orbit, so the loops drift and never repeat in the same place. The result is the least visible scratch pattern of the three, and a removal rate that sits between the rotary and the finishing sander — slower than a rotary, but honest: what raking light shows is what the finish will show.

Edges and control. An orbital pad runs flat on the work and is guided rather than held back; it tilts only if you let it, and its habit at an edge is to roll over and soften the arris rather than gouge it. A rotary machine pulls in the direction it spins, wants two hands, and a tilted rim meeting an edge takes a bite out of it in an instant. On a veneered or thin panel that difference is whether the surface survives.

Which to choose. Reach for the rotary when there is a lot of material to shift and the surface will be worked again afterwards: stripping paint, fairing a curve, shaping, levelling a top that is well out of flat. Reach for the random orbital for anything that will take a finish, for the whole grit ladder below, and for anything veneered or thin. Where a rotary has been over a surface, treat its arcs as the starting scratch pattern: drop back a rung on the ladder and let the orbital erase them before climbing. Dual-mode machines that switch between a forced-rotation gear drive and a free random orbit behave like a rotary in one setting and an orbital in the other, and the advice above follows the mode, not the badge on the machine.

§2 The ladderThe grit ladder, one rung at a time

Sanding is a sequence of scratch patterns, each grit erasing the coarser scratches of the one before and leaving finer scratches of its own. This site's tools work along the ladder40 · 60 · 80 · 120 · 150 · 180 · 240 · 320 — a workshop rule of thumb, the same one theSanding Disc Estimator publishes as its constants, not a standard. Two habits make any ladder work:

• Start as fine as the surface allows. The starting grit answers to the surface in front of you — sawn or weathered stock needs a coarse start, planed or pre-sanded stock can begin much further up. Starting coarser than the surface needs digs scratches you then spend three grits removing.
• Climb rung by rung. Skipping a rung leaves the previous grit's scratches at the bottom of the new grit's pattern, invisible until a finish makes them permanent. Each grit's job is only to remove the scratches of the one before; that job done, move on — polishing away at one grit flattens the abrasive, burnishes the wood and gains nothing.
• Stop where the finish wants. There is a point beyond which further grits close the surface against the finish you plan to apply. The estimator's own targets stop at 150 for painted work, 180 for a standard clear finish and 240 for fine work — rules of thumb again, adjustable in the tool.

How many discs a job needs at each rung is arithmetic, not intuition — theSanding Disc Estimator plans the sequence for your surface and finish and counts the discs, spares included and labelled.

§3 The handMovement and pressure

A random orbital sander is designed to do its cutting under little more than the weight of the machine and a guiding hand. Pressing harder does not cut faster: it slows the pad's orbit, turns random scratching into deep looping scratches, heats the disc so it clogs, and wears the abrasive in patches. If the machine is not cutting fast enough, the answer is a coarser grit or a fresh disc — never more force.

• Keep the pad flat on the work; tilting onto an edge digs a gouge in one pass.
• Move in slow, steady, overlapping passes, letting each pass lap well over the last, and cover the surface in an even pattern rather than orbiting your attention around the spot that looks worst.
• Racing the machine across the surface leaves it no time to erase the previous grit's scratches; a slow, deliberate rate of travel is the single easiest upgrade to most people's sanding.
• Start the sander on the work (or lower it running with care) and lift it before you stop it, so a spinning-down pad never sits parked on the surface.
• Between grits, vacuum the surface or wipe it clean — grit carried forward scratches below the new grit's pattern.

§4 SwirlsWhere swirl marks come from

The little pigtail swirls that appear under stain or lacquer have a short list of causes, and all of them are avoidable: skipped rungs on the ladder; pressing hard enough to stall the random action; tilting the pad; dust or a shed abrasive grain trapped between disc and work; a worn disc cutting unevenly; and moving the machine too fast for the grit to finish its work. The defence is the routine above plus one habit at the end: inspect the surface in low, raking light — a lamp laid nearly parallel to the work shows every swirl and missed patch before the finish does, while there is still time to drop back a grit and erase them.

§5 DustDust control — the HSE-sourced part

Fine sanding dust is the most dangerous dust a small workshop makes, and an orbital sander makes little else. This section is not workshop folklore: each fact below is the Health and Safety Executive's published guidance on wood dust.

• Wood dust can cause serious health problems, including asthma, and hardwood dust can cause cancer, particularly of the nose.¹
• Sanding is a very dusty job that may need additional protection: a suitable face mask as well as dust extraction.¹
• Never sweep up wood dust and never clear it with compressed air — both simply put the finest dust back into the air you breathe.¹
• Clean up with an industrial vacuum cleaner of at least dust Class M.¹

In practice that means the sander runs connected to extraction through its dust port, a suitable mask goes on before the trigger is pressed, and the machine's own perforated pad and hood do their work with a hose attached rather than a clip-on bag alone. The estimator's page carries the fuller HSE-sourced dust panel, including exposure limits and extraction examination requirements, alongside its disc arithmetic.

§6 RoutineThe routine, in one list

• Choose the machine for the job: rotary only where the surface will be worked again; random orbital for anything that will take a finish.
• Read the surface and choose the starting grit it needs — no coarser.
• Connect extraction, put the mask on, and start on the work.
• Slow overlapping passes, flat pad, the machine's weight only.
• Finish each grit everywhere before moving one rung up; vacuum between rungs.
• Check in raking light before calling any grit done.
• Clean up with the Class M vacuum — never the brush, never the airline.¹