How to finish wood: stain vs oil vs polyurethane

Stain is color, not protection

Stain is pigment or dye in a carrier. It changes the color of the wood but leaves little protective film. A stained piece still needs a clear topcoat unless the stain is a combined stain-and-finish product. Test stain on an offcut from the same boards — the same can looks very different on oak, pine and maple.

Some woods — pine, cherry, maple, birch — absorb stain unevenly and go blotchy. A pre-stain conditioner, a gel stain, or a thin coat of dewaxed shellac before staining helps. Many woodworkers simply leave cherry and walnut unstained.

Buy a little extra of the same stock for test pieces. If you are buying hardwood by the board foot, one spare board of the kind worked out in Board feet in a 4/4 hardwood board, 8 in wide and 8 ft long gives you offcuts for every stain and topcoat test, sanded to the same grit as the real parts.

Choosing a topcoat

For a dining table or desk, a film finish like polyurethane or varnish is the practical choice. For a decorative box or a chair that is handled, an oil finish is easy and easy to repair. Outdoors, use products made for exterior use — interior polyurethane breaks down in sunlight.

Surface prep decides the result

1. Plane or scrape out machine marks first; sanding alone takes a long time to remove them.
2. Sand through the grits in order — for example 120, 150, 180 — without skipping. Each grit removes the scratches from the previous one.
3. Stop at 180–220 for most finishes. Sanding finer can keep stain from penetrating evenly.
4. Remove all dust with a vacuum and a lightly dampened cloth or tack cloth.
5. For water-based finishes, wipe the wood with a damp cloth, let it dry, and lightly sand again to knock down raised grain.

Applying the finish

• Stain: wipe or brush on, let it sit for the time on the label, then wipe off the excess across and along the grain. Let it dry fully before topcoating.
• Oil: flood on, let it soak for the labeled time, wipe off every bit of excess. Repeat over several days for more depth.
• Polyurethane: stir, do not shake. Brush thin coats with the grain, keep a wet edge, and let each coat dry per the label. Scuff-sand lightly with 320 grit between coats and clean off the dust.
• Wipe-on: apply thin coats with a lint-free rag. Each coat is thinner than a brushed coat, so plan on more coats.
• Undersides and inside surfaces: finish them too, with at least a coat or two. It slows moisture exchange and helps keep wide parts from cupping.

No finish stops wood from moving. A film slows how fast moisture gets in and out, but over a season a solid top still swells and shrinks across its width, finished or not. How much a 36 inch red oak tabletop moves puts a number on it (a little over half an inch), and How much a 900 mm walnut tabletop moves does the same in metric. Fasten tops so they can move, and finish both faces the same way so one side does not dry faster than the other.

How much finish do you need?

Coverage is printed on the can, usually as square feet per gallon or quart. Typical figures are around 400–600 sq ft per gallon for interior stains and brushed polyurethane on sanded wood, and much less — sometimes 100–200 sq ft per gallon — for exterior stain on rough or weathered boards. Multiply your area by the number of coats, divide by coverage, and add a little for what stays in the brush.

Two worked examples go through the arithmetic step by step: How much polyurethane for a dining table (three coats on the top, fewer underneath) and How much stain for a 12x16 deck (two coats on weathered boards, railings extra). The coverage on your can is always the number to use; the figures here are only for planning.

Shelves, pine and MDF

Bookcases are often built from pine boards or MDF, and both need a little care. Pine blotches under most stains, so use a conditioner or gel stain, or skip the stain and use a clear finish. MDF has no grain to stain at all: it is meant to be painted, and its cut edges soak up primer and paint like a sponge. Seal the edges first with a coat of primer made for MDF or a thinned coat of shellac, sand lightly, then paint.

A finish adds nothing to a shelf’s stiffness, so check the span before you build. Will a 1x12 pine shelf sag over 36 inches and How much an 18 mm MDF shelf sags over 80 cm show how quickly a loaded shelf passes the point where the sag is visible, and what a hardwood front strip does about it.

Cure time and use

Dry to the touch is not cured. Most film finishes need days to weeks to reach full hardness; the label gives the time. Avoid setting heavy objects, rubber feet or wet glasses on a new finish until then. Many cured finishes are generally considered safe for food contact once fully cured, but for cutting boards and butcher blocks, a food-grade mineral oil or a product labelled food-safe is the usual choice.

Exterior wood

Decks and fences take a beating from sun and water, so they need exterior stains or sealers and regular recoating — often every few years depending on the product and exposure. Clean the wood, let it dry, and apply in the conditions the label specifies. Weathered wood drinks much more stain than new wood, so estimate generously. If the deck is still at the planning stage, How many deck boards for a 12×16 deck counts the boards, and the stain for that same deck is worked out in How much stain for a 12x16 deck.