Why any finish changes the look
The Forest Products Laboratory explains that almost any clear material — oil, wax, varnish — bends light more like wood than air does, so once it fills the surface cells the eye sees deeper into the wood. That is why oiled or varnished wood looks richer than bare. It is also why even a clear finish changes how a board looks; test it on an offcut first.
Film-forming and penetrating
The Laboratory divides finishes into two families. Film-forming finishes — varnish and polyurethane, lacquer, shellac and paint — build a layer on the surface. Penetrating finishes — oils and penetrating stains — soak in and leave none. Its Wood Handbook notes that films are easier to clean, resist staining better and protect against scratches and dents, but eventually wear, crack or peel; penetrating finishes cannot peel, but outdoors they last a fraction as long.
| Family | Film? | Examples | Cleanup | Repair |
|---|---|---|---|---|
| Polyurethane and varnish | Yes | Minwax Fast-Drying Polyurethane; GF High Performance | Mineral spirits (oil-based) or water (water-based) | Sand lightly and recoat |
| Lacquer | Yes | Minwax Clear Brushing Lacquer | Lacquer thinner | New coats melt into earlier ones (Minwax) |
| Shellac | Yes | Zinsser Bulls Eye Shellac | Denatured alcohol or ammonia and water | Recoat |
| Paint | Yes, opaque | Primer and enamel | Depends on the base | Sand, prime bare spots, recoat |
| Oils and oil blends | No | Linseed and tung oil; Watco Danish Oil | Mineral spirits | Clean and re-apply |
| Hardwax oil | No film, per its maker | Osmo Polyx-Oil | Osmo's brush cleaner | One coat over the cleaned surface |
| Wax | A thin layer | Minwax Paste Finishing Wax | Solvent-based (mineral spirits) | Re-apply and buff |
Classification after the Forest Products Laboratory; products and cleanup as each maker states. Osmo says its oil leaves a microporous surface that will not crack or peel.
Choosing between them
- Tabletops, cabinets, floors: the Laboratory points to films for surfaces that need cleaning and wear resistance, thicker and harder ones for floors. See polyurethane or shellac and lacquer.
- A natural, in-the-wood look, easy repairs: penetrating finishes. Worn spots can be patched without lap marks, the Laboratory notes. See oils.
- Colour: stain first, under either family — see wood stain. For an opaque surface, paint and primers.
- A low sheen over stain, on a surface that sees little wear: wax, which Minwax describes as for stained surfaces that do not need the hard coat of polyurethane. See wax.
- Items that touch food: see food-safe finishes before choosing.
For a particular piece — a tabletop, a cutting board, outdoor work, or walnut and cherry, where colour matters — how to choose a wood finish works through the choice by use.
Outdoors
Outdoors the trade-off sharpens. The Laboratory explains that film-forming finishes peel from wood the sun has already degraded unless it is sanded back to sound wood, while penetrating finishes do not peel but need renewing far more often. Minwax says of its spar urethane that no clear exterior finish will permanently protect wood from sun, snow, ice and rain, and that it should be checked every year. Deck and siding finishes are a separate class of product, not covered in this guide.
Note— Labels are not a standard
The Forest Products Laboratory notes that makers do not follow a standard set of definitions: 'sealer', 'oil' or 'varnish' on a can can mean different things from different makers. Read the data sheet for what the product is and how it behaves.
Choosing a type of finish
- How the surface will be used, cleaned and worn
- Film or penetrating, for the look and the repairs you want
- Indoors or out
- Whether it touches food
- What it must go over — stain, filler or an old finish
- The product's data sheet, not just the label
Sources
The film/penetrating division and its consequences are the Forest Products Laboratory's. Product examples are named makers' own descriptions. No family is ranked for durability here: performance depends on the product, the coats and the use, and the makers' claims are their own.
- 1.
Christopher G. Hunt. Wood Handbook: Wood as an Engineering Material (FPL-GTR-282), Chapter 16: Finishing Wood. USDA Forest Service, Forest Products Laboratory, 2021.
Used for: Film-forming and penetrating finishes and their differences; films easier to clean and better against scratches and dents, thicker films for floors; penetrating finishes not peeling but lasting less long outdoors; any clear finish letting the eye see deeper into the wood; interior clear films lasting essentially indefinitely; makers not sharing standard definitions.
- 2.
Mark Knaebe. Finishes for Wood Bowls, Butcher Blocks, Other Items Used for Food, and Children's Toys (FinishLine). USDA Forest Service, Forest Products Laboratory, 2013.
Used for: Varnish, lacquer and shellac as film-forming finishes that eventually chip, peel or crack; drying oils patched without lap marks.
- 3.
Minwax Product Guide. Minwax.
Used for: Polyurethane, lacquer and spar urethane as clear protective films; wax over polyurethane preventing recoating; no clear exterior finish permanently protecting wood.
- 4.
Technical Data WAT-02, Watco Danish Oil. Rust-Oleum (Watco), 2020.
Used for: Danish Oil as a penetrating oil-and-varnish blend that hardens in the wood.
- 5.
Product Information, Polyx-Oil Original. Osmo, 2020.
Used for: Polyx-Oil as a hardwax oil whose maker describes a microporous surface that will not crack, peel, flake or blister.
- 6.
Technical Data BEY-06, Zinsser Bulls Eye Shellac. Rust-Oleum (Zinsser), 2021.
Used for: Shellac as a fast-drying film finish.
- 7.
Technical Data Sheet, High Performance Topcoat. General Finishes, 2019.
Used for: A water-based topcoat cleaned up with soap and water.
- 8.
Technical Data Sheet, Minwax Paste Finishing Wax. Minwax.
Used for: Paste wax for stained surfaces that do not need the hard protective coat of polyurethane.
About this guide
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