Repairing finish damage

Overview

Most damage to a cured finish can be repaired without stripping if you choose the lightest method that works. A repair only disappears when it matches the surroundings in colour, body, texture and sheen (Scott Doyle); a perfect colour match with the wrong sheen still shows. Start with the least invasive fix and practise on scrap. Where the damage is general, see Stripping and refinishing.

When to use

• A surface scratch, deep scratch or gouge, a white ring, a small chip, or crazing on shellac or lacquer.
• Not for catalyzed finishes: Flexner says conversion varnish resists solvents and abrasives, so rings and heat marks may need a strip.
• Not for valuable antiques; House Painting Info advises appraisal before altering an original finish.

Materials and tools

Burn-in or wax fill sticks in matching colours, a burn-in knife and heat source, touch-up markers, 400 wet/dry paper, 0000 steel wool, denatured alcohol, pumice and rottenstone, mineral oil, padding lacquer or shellac.

Steps

Light scratch. Fine Woodworking: a surface scratch may rub out with steel wool, but usually not. Rubbing changes sheen, so you may need to rub out the whole surface (Rubbing out a finish).

Deep scratch: burn-in.

1. Heat the knife tip with the concave side toward the flame so soot does not contaminate the resin; judge the heat on scrap. Too hot and the resin bubbles and smokes; too cool and it forms drops that re-solidify (Bütz).
2. Melt the shellac stick into the scratch, filling as little of the surrounding area as you can; each hot pass puts the finish at risk (Doyle).
3. Level with 400 wet/dry kept wet, taking care not to cut through the finish, then polish (Doyle).
4. Match colour with stain and a small brush, and touch in the finish (Bütz). Burn-in works on oil, varnish, shellac and lacquer; polyurethane and other plastic finishes sometimes blister (Bütz).

Easier fill. Wax sticks and putty are simple but compromise the result (Doyle). Mohawk describes its Quick Fill burn-in sticks as wax-based, applied with a simple heat source then leveled, no sandpaper.

Touch-up. General Finishes sells oil-based stain markers for scratches, watermarks and worn edges, and a fill-stick assortment.

White ring (Woodcraft, from Foolproof Wood Finishing). Dampen a rag lightly with denatured alcohol and wipe; then pumice with mineral oil, then rottenstone. If you cut through the topcoat you must replace colour and topcoat. Flexner adds that abrading works on all finishes except very hard catalyzed ones.

Crazing. Reamalgamation: apply an amalgamator (a solvent blend) with a pad so the softened finish redistributes smooth. House Painting Info says it usually works on shellac or lacquer, rarely on varnish, and needs skill.

Common problems

• Repair shows as a different sheen. Cause: abrasion or fill sheen differs. Fix: rub out the whole surface or add a final coat.
• Blistered or bubbled burn-in. Cause: knife too hot, or polyurethane under it. Fix: lower heat; test elsewhere.
• Through-sanding to bare wood. Fix: recolour and recoat locally.

Safety and environment

An alcohol lamp or hot knife is an ignition source; Bütz warns never to leave it burning unattended. Denatured alcohol is flammable; keep it away from the lamp. Ventilate. See Oily rag safety for oil-soaked rags.

Related

• Troubleshooting finish defects: diagnosing what went wrong.
• Stripping and refinishing: when a repair is not enough.
• Shellac, Nitrocellulose lacquer, Paste wax: repairable finishes and wax top-ups.
• Furniture polish and finish revivers, Sheen control and matching
• Curing conditions and time-to-use: let new finish cure before use.