Runs, Nibs, Cloudiness and Fisheye: Fixing a Finish That Went Wrong

A RUN or SAG is too much finish in one place. A DUST NIB is a speck that landed while it was wet. Both are lumps on an otherwise sound film, and both are fixed the same way. Wait until the finish is genuinely hard — days for oil-based polyurethane, not hours. Attacking a run while it is soft drags and smears it into something much worse. Then slice the top off carefully with a sharp blade or sand it flat with a block, and recoat. Use a block, never fingers. Fingertips follow the lump and leave a dip around it, which is then visible from every angle under a gloss finish.

A milky haze in the film is usually moisture trapped in it — from applying in high humidity, or in a cold room, or over a surface that was not dry. It is called BLUSHING. With shellac and lacquer it often clears by re-wetting the surface with the solvent so the moisture can escape as the film re-forms. With polyurethane it generally does not clear and the coat has to come off. A white RING on a waxed or shellacked surface is the same thing locally — moisture in the finish, not in the wood. It usually responds to gentle heat, or to abrading and re-waxing. A BLACK mark is different: that is water that reached the wood and reacted with tannin, and it has to be bleached or sanded out.

FISHEYE is small craters where the finish pulls away from the surface. The cause is contamination — almost always silicone, from a furniture polish or a spray, and it is in the wood rather than on it. The reliable cure is a barrier coat of dewaxed shellac, which sticks where other finishes will not. WRINKLING means a coat was applied too thickly, or recoated before the one beneath had cured, so the solvent attacked it. It has to come off; there is no way to flatten it. A finish that NEVER DRIES has three usual causes: oil-based finish over a not-yet-cured oil, stain that was not wiped off, or old shellac past its shelf life. Only the first can wait it out — the other two must be removed.

Strip when the problem is IN the film across the whole surface — wrinkling, widespread fisheye, a coat that will not cure — rather than on it. Local faults are local repairs. Shellac lifts with alcohol, which is why it is so forgiving. Oil-based varnish needs a chemical stripper or scraping and sanding. Sand as little as possible on shaped work: a moulding sanded to remove a finish loses its crispness, and that is a worse outcome than the finish was. Most of these faults trace back to three habits, and all three are cheap: test the whole schedule on an offcut of the same board, apply thin coats, and let each one cure before the next. Almost nothing in this rung happens to someone doing those three things.