Rubbing out a finish

Overview

Rubbing out is the work after the last coat: sanding a cured film flat, levelling dips and humps, then polishing to the sheen you want. Woodcraft's author says even a flawless application may come out too dull or too shiny, and rubbing out both fixes that and removes defects. Sheen is controlled by scratch size: coarse scratches (400 or 600 grit) give a flat look, finer scratches satin, and a scratch-free surface gloss.

When to use

• Hard film finishes: lacquer, shellac, varnishes, polyurethanes; thicker coats give more latitude. Wipe-on varnish can be rubbed out only less aggressively, to remove defects or change sheen.
• Not on thin penetrating finishes such as pure oils and oil-varnish mixes, which give no film to rub (Woodcraft). See Hardwax oil and Tung oil.
• Let the film cure first; see Curing conditions and time-to-use. In a pumice-and-rottenstone sequence, Fairchild waits 48 hours after the last fine pumice pass before rottenstone.

Materials and tools

• Abrasive papers, 400 and 600 grit for levelling. Stearated paper can be used dry; use silicon-carbide wet/dry with soapy water, rubbing oil or mineral spirits for lacquer, shellac and solvent varnish, which clog stearated paper (Woodcraft). Stearated is preferred for water-based finishes. Wet-sanding lubricant: a capful of dish soap in a pint of water.
• Cork or felt-backed block; 0000 steel wool or gray abrasive pads; Micro-Mesh or 1000 to 4000 grit abrasive discs.
• Pumice (2F to 4F) and rottenstone with rubbing oil, or two-step automotive compounds.

Steps

1. Thick film (6 to 10 coats): wrap 400 or 600 grit round a block and sand only enough to flatten, removing nibs and drips. Don't wet-sand profiled edges. A few shiny spots remaining is acceptable (Woodcraft).
2. Level with the next finer grit until the surface is mostly dull and scratch-patterned, then use steel wool to blend to a consistent flat sheen.
3. Satin: rub with 0000 steel wool and a lubricant, three passes then refold, repeating to about twelve passes; go lightly on edges. Woodcraft puts wax thinned 50/50 with mineral spirits between satin and gloss.
4. Gloss: go through grits to 2000 and polish with a coarse then a fine compound. Wood Magazine's polyurethane advice is pumice (2F and 4F) for satin and rottenstone for semigloss; use one or the other, not both, because coarse pumice leaves scratches coarser than 2000 grit.
5. Pumice work: use a lubricant, rub with even pressure and stroke count, avoid circles, check often, and watch edges (Fairchild). Use oil, not water, on shellac, since water turns it white.
6. Thin finish: start with 600 grit, hand-backed, dry-sand along the grain until brush strokes and nibs go (usually one or two passes) and hold paper flat at edges (Woodcraft). Pekovich's wiping-varnish method builds a little glossier than needed, then steel-wools to a dull even sheen before wax.
7. Wipe clean, removing oil film with naphtha or dewaxer; inspect in raking light.

Common problems

• Rub-through at edges: too much pressure; touch up as described in Repairing finish damage.
• Witness lines: cutting through one layer into the next, on finishes whose coats don't fuse, like varnish and water-based (Woodcraft).
• Paper clogs: finish not dry enough, or wrong paper for the finish.
• Swirls: circular strokes with abrasive powder.

Safety and environment

Mineral spirits and naphtha are flammable: ventilate and keep ignition sources away. For used rags and steel-wool pads, see Oily rag safety.

Related

• Sheen control and matching: choosing the target sheen.
• Build coats and sanding between: building enough film to rub.
• French polish and padding: rubbing out shellac.
• Wipe-on application, Brush application: producing the film.