The first thing most people do when a finish is damaged is apply more finish. This is almost always wrong.
What you add changes based on what's already there. Get it wrong and you've turned a surface scratch into a refinishing job. So: stop. Test before you touch anything.
Three swabs is all it takes.
Denatured alcohol on a cotton swab, inconspicuous spot. If the finish softens or smears, it's shellac. Shellac is the most repairable finish in woodworking — fresh shellac re-dissolves into old shellac. The two coats bond chemically. A good repair disappears.
Mineral spirits next. If the finish gets tacky or lifts, it's wax. Wax over wax repairs easily. Wax under a film finish is a different problem and needs to come off before anything else goes on.
Lacquer thinner last, and carefully. Wrinkling or dissolving means lacquer. Lacquer repairs well with the right product and proper ventilation.
If none of the three causes a reaction, you're probably looking at polyurethane or waterborne acrylic. Poly is the hardest finish to spot-repair invisibly; more on that below.
Once you know what's there, the strip-vs-repair decision is straightforward.
Strip the whole piece when the finish is peeling or crazing in multiple locations — spot repair at that point is whack-a-mole. Strip when you find incompatible products layered over each other: oil-based over shellac, lacquer over waterborne, wax over bare wood before a film finish. These lose adhesion in ways no patch will fix. Strip when your identification tests come back ambiguous and the stakes are high. On a piece with real value, don't gamble.
For isolated damage where you know what's on the surface, repair in place.
The sequence depends on the finish.
Shellac: clean the damaged area with denatured alcohol, apply fresh shellac in thin cuts, build up layers. Shellac dries in minutes. Feather the edges. Evaluate under raking light for sheen match, then level and polish as needed. This is the most forgiving repair in the shop.
Oil finishes: scuff the damaged area lightly at 320-grit — enough to open the surface, not enough to pull color from the wood. Apply fresh oil, same product if you have it, and wipe off after the appropriate open time. Do not leave it sitting. On an actively maintained piece, new oil blends well. On a piece that's been sitting dry for years, expect some variation; it will normalize over time. Cold-shop note: below 5°C, oil won't cure. Spreading it on and waiting for it to dry is waiting for nothing. Schedule the repair for when the shop is warm.
Waterborne finishes: feather the edges. Sand into the damaged area and extend several inches outward in light, diminishing passes. You're creating a ramp from the damage to the intact surface. Apply thin coats and feather each one outward. Raking-light check after every coat. You're looking for a blended sheen transition, not a hard edge where the new coat stops.
Polyurethane: the hard case. Your options are sanding deep enough to get below the damage and feathering the surrounding film, refinishing the whole panel, or accepting a visible repair. Wipe-on poly is more forgiving for spot work than brushed — the thinner film blends more naturally. But invisible poly repair isn't always achievable. Tell the client that plainly.
Watermarks under oil: try mineral spirits on a cloth first. Many watermarks are surface contamination, not finish failure. The mineral spirits absorbs the pattern; when it evaporates, the surface often looks normal. If not, light sanding and re-application is the repair.
Wax buildup: strip with mineral spirits, allow to dry completely, re-wax thin. Buffing built-up wax just moves the problem around. Strip it.
One rule covers every step: evaluate under raking light. A single warm-white LED at 15 to 30 degrees off the surface shows you sheen inconsistencies, trapped dust, and blend quality. Ambient overhead light sees nothing useful. If a repair looks acceptable under ambient light but wrong under raking light, it will look wrong in a window.
The goal is a repair that disappears. That starts with knowing what you're working on before you open a can.