# Finish Repair and Refinishing Hewn Shop Notes · https://hewn.build/shop-notes/finish-repair-and-refinishing AI-drafted, not yet checked: Drafted by Chip, the writing system Hewn runs on Claude, from Hewn's craft corpus. Jonathan Salomons has not read this note yet, and no outside reader has checked it. Read it as a shop note: check a number before you cut to it. Cite as: "Finish Repair and Refinishing." Hewn Shop Notes, hewn.build/shop-notes/finish-repair-and-refinishing. Drafted 2026-05-05; AI-drafted, not yet checked. --- Most people try to fix a finish before they know what it is. That's the mistake. It's not laziness — it's optimism. You see a scratch, you grab the same finish you used last month, you lay on a coat, and three days later you have a visible repair plus a delamination problem and a whole new thing to explain to the client. Identify the finish first. Then decide whether to repair or refinish. Then pick up a tool. --- **Three tests that tell you what you're dealing with** The solvent test is the most reliable. Dampen a cotton rag and hold it against an inconspicuous area for 10–15 seconds. Denatured alcohol dissolves shellac. If the rag picks up finish color, you have shellac. Lacquer thinner dissolves lacquer — try it after the alcohol test. If neither dissolves anything, you're looking at polyurethane, varnish, or a hardened oil system. Oil-based and water-based poly respond the same way to solvents once cured: they don't. If you can't tell them apart by this test, treat them as the same family for re-coat purposes. The scratch test adds context. A coin dragged firmly across shellac leaves a clean white powder. Lacquer scratches similarly but resists a bit more. Cured polyurethane flexes before it yields — the scratch looks glossy rather than powdery. Age helps, but don't rely on it alone. Pre-1960s furniture is almost certainly shellac, varnish, or oil-based paint. Warm amber with a slightly soft feel points to shellac or early oil varnish. A surface that crazes or spiderwebs like old paint is nitrocellulose lacquer. High-gloss plastic clarity is modern poly. If you run all three tests and still can't identify the finish — or if the piece has been refinished multiple times by people who also didn't know what they were doing — strip it. Coating an unknown substrate is how you end up with fisheye and peeling. --- **Spot repair or full refinish** Here's the question people get wrong by defaulting to spot repair when the piece calls for a full refinish. They end up with a repair that looks worse than the original damage because it halos. Spot repair works when the damage is small (under 10–15% of the surface), when you've identified the finish with confidence, and when the surrounding film is in good shape — no widespread crazing, no flaking, no adhesion failure. If you're not sure the repair will blend, do a test: lay a thin fresh coat over an intact, inconspicuous section and let it cure. If it halos, the whole surface needs attention. Full refinish is the call when damage is large or centered in a high-visibility zone, when the existing finish is unknown or layered incompatibly, or when the film is failing across the board. Spot repairs don't hold in a failing finish. That's a rule. --- **Stripping** Chemical strippers are the right tool for furniture refinishing. NMP-based products like Citristrip are slower than methylene chloride but safer and adequate for most shop work. Apply the stripper thick — don't brush it back and forth, just lay it on. Cover with plastic wrap to slow evaporation. Wait until the finish wrinkles and lifts. Pull it off with a plastic scraper, repeat if needed, finish with a mineral spirits or alcohol wipe to remove residue. Inspect in raking light before touching a piece of sandpaper to it. Heat guns are for paint on outdoor furniture. Not for clear-coated interior pieces. They discolor wood, raise grain unpredictably, and the risk-to-benefit ratio is wrong. Same for sanding as a primary stripping method on thick poly builds — you'll start at 60-grit and work up, which introduces surface damage that takes real time to clean up. --- **Re-coat compatibility** Shellac over shellac: re-coat direct. It resolves into the previous layer. Most forgiving situation in finishing. Lacquer over lacquer: the solvent in fresh lacquer re-dissolves the surface of the cured coat. The new material fuses chemically with the old. Spot repair on lacquer done right is invisible for this reason. Oil-based poly over oil-based poly: scuff-sand 220–320, wipe with mineral spirits, let dry, re-coat within the recoat window. If the existing coat is old, test first — if any finish picks up on a mineral spirits rag, lay down a dewaxed shellac barrier before new poly. Water-based poly: same protocol as oil-based. The hard rules: nothing goes over wax. Strip it, neutralize with naphtha or mineral spirits, and start over. Oil-based poly over pre-waxed shellac has the same problem — the wax in standard shellac kills adhesion. Use dewaxed shellac (SealCoat or equivalent) as your barrier coat. Lacquer over oil-based poly or varnish will not adhere. It only goes over cured lacquer or dewaxed shellac. --- **Touch-up** Water rings and white spots in shellac or lacquer are moisture trapped in the film — not damage to the wood. Light sand at 400, re-coat with the same finish. In shellac, a pad of dilute shellac rubbed across the ring can dissolve it without sanding. For scratches in the film: clean and dry, apply the same finish by brush tip into the scratch only, cure fully, level with 400–600 wet-dry used dry, then 0000 steel wool, buff with paste wax. For deep scratches through to wood, color-match the wood first, seal, then build the film back with thin coats. The most common reason a technically correct repair is still visible: sheen mismatch. A matte repair in a satin field reads immediately. Test sheen in an inconspicuous area before committing. If sheen won't match on the repair alone, coat the whole surface. --- Identify before you repair. Re-coat to the rules of the finish you've identified. Test the blend before you commit to spot repair. That's all it is. The failures almost always trace back to the first step.