Troubleshooting finish defects

Overview

Most finish defects are one of three things: a contaminated surface, a coat applied too thick or too fast, or weather. Identify the finish first, because shellac and lacquer re-dissolve in their solvents and can be repaired in place, while varnish and catalyzed finishes cannot. The advice below is mostly Bob Flexner's "baker's dozen" (Popular Woodworking, 2014) plus the other cited sources.

When to use

• A wet or just-dried coat looks wrong and you want to decide whether to wait, sand or strip.
• For damage to a cured finish in use, see Repairing finish damage; for removing it all, Stripping and refinishing.

Materials and tools

400 grit and 0000 steel wool, the correct thinner, lacquer retarder (butyl cellosolve for shellac), fish-eye eliminator, denatured alcohol, a scraper.

Steps

1. Stop applying. Fresh defects are easiest to fix wet; once dry, sand level.
2. Name the finish: Shellac and Nitrocellulose lacquer fix in place with solvent; varnish and polyurethane are sanded.
3. Match the symptom below, fix, then recoat thinner and in better conditions (Curing conditions and time-to-use).

Common problems

• Fisheye (craters). Cause: silicone from furniture polish. Fix: sand level and add fish-eye eliminator (thinned with mineral spirits for varnish; emulsified for water-based) to every later coat, or strip. Prevention: wash with solvent, ammonia or TSP, or seal with shellac.
• Orange peel (sprayed). Cause: finish too thick, pressure too low, gun too fast or too far. Fix: sand level, thin the next coat or adjust technique. See Spraying with HVLP, Thinning and viscosity.
• Blush (milky haze). Cause: humid day with lacquer or shellac. Fix: wait, mist retarder or rub with fine steel wool.
• Runs and sags. Cause: coat too heavy. Fix: sand or scrape level; brush out while wet; thinner coats.
• Bubbles and pinholes. Cause: brushed varnish in hot weather; air escaping open pores in oak or mahogany. Fix: tip off, add 5-10% mineral spirits, sand level and recoat thinned; for spray, "dust" with near-dry coats first or fill pores (Grain filler (pore filler)).
• Brush marks. Cause: finish too thick. Fix: thin, sand level, recoat. See Brush application.
• Cotton blush (white fuzz in lacquer). Cause: cleanup-grade lacquer thinner. Fix: sand out and recoat with proper thinner.
• Blotching (stain). Cause: uneven absorption in maple, cherry, pine, birch, poplar. Fix: prevention only: conditioner minimises it but makes dark colour harder (Wood Magazine; Popular Woodworking). See Pre-stain conditioner and washcoat, Finishing resinous and blotchy woods.
• Crazing and alligatoring. Cause: Flexner attributes cracks to wood movement and finish ageing from oxidation and UV, and notes that thicker built-up coatings crack more. Ackerman describes alligatoring as broad plate-like fragments in built-up layers; surface checking as hairline cracks following the grain. House Painting Info lists thick coats, an undried coat under the next, mixed finishes and sun as causes. Fix: reamalgamation can work on shellac and lacquer, rarely on varnish; otherwise strip.
• White rings. Cause: water in the film. Fix: lightly dampened denatured alcohol pad (shellac, sometimes lacquer), then pumice in mineral oil and rottenstone; abrading nearly always works but changes sheen. Flexner says catalyzed varnish will not dissolve or abrade. If you cut through the topcoat, recolour and recoat (Woodcraft).
• Sticky or tacky film. Cause: cold, thick coats, oily wood, old shellac. Fix: warmth, or strip. For oily woods wipe with acetone, lacquer thinner or denatured alcohol and seal with dewaxed shellac (Target Coatings). See Degreasing oily woods.
• Fuzzy grain. Cause: water-based product or stain raising fibres. Fix: dampen, dry, sand; see Raising the grain.

Safety and environment

Solvents named here are flammable; ventilate and keep ignition away. Rags soaked in oil finishes can self-heat; see Oily rag safety. Fish-eye eliminators and retarders are chemicals: read the can.

Related

• Sanding schedule, Build coats and sanding between, Rubbing out a finish
• Repairing finish damage, Stripping and refinishing
• Glue squeeze-out and common finish failures: glue-caused splotching.
• Wipe-on application, Spraying aerosol finishes