Finishing is where a woodworking project either comes together or falls apart. You can pick a beautiful slab, build a solid piece, and still end up with a top that stains, peels or wears through in a few months. It usually comes down to picking the wrong finish for the job.
This series covers what we teach in our classes and what we tell everyone who works in our shop. Part 1 starts with the most important decision: what type of finish to use, and how much protection you can really expect from it.
The M2 Finishing Guide
- Film vs. Penetrating Wood Finishes (you are here)
- Oil Finishes and Hard Wax Oils
- How to Sand for a Smooth Finish
- How to Apply Wood Finish
Shopping for finished furniture rather than building it? Our Finishing 101 post explains what to look for.
The Two Main Types of Wood Finish
Nearly every finish on the shelf falls into one of two categories: film finishes and penetrating finishes. Knowing which one you're using changes how you prep the wood, how you apply it, and how much protection you get.
Film Finishes: A Physical Barrier on Top of the Wood
A film finish is anything that sits on top of the wood and builds a physical barrier between the wood and everything around it. Common examples are polyurethane, varnish, lacquer and shellac, in oil-based or water-based versions. Epoxy is the heavy-duty version of the same idea.
These finishes go on as a liquid made of resins dissolved or suspended in a solvent. As the solvent evaporates and the finish cures, what's left behind is essentially a thin layer of plastic. That layer keeps water, spills and stains from ever reaching the wood, which is why a good film finish gives you a robust, durable top coat that, for most everyday use, is close to bulletproof.
Penetrating Finishes: Soaked Into the Wood
A penetrating finish soaks into the wood instead of building up on top of it. This includes boiled linseed oil, tung oil, Danish oil, teak oil, waxes and hard wax oils. Some of them do harden; hard wax oils, for example, contain hardeners that make them much firmer once they cure. But they still don't build a real layer on the surface. They live in the top layer of the grain, so any film you get is very thin, and the wood surface itself is still pretty exposed.
Why Oil Finishes Look So Good
Here's what penetrating finishes do better than anything else: they bring out the color and tone of the wood. Side by side on bare wood, most people agree an oil or hard wax oil makes the grain richer and deeper than a film finish alone. That's because they soak deep into the wood fibers, and that's where the color and depth come from. They're also easy to apply and give a warm, natural, hand-rubbed look.
The Truth About Durability
Here's the part people don't always like to hear: film finishes are much more durable than penetrating finishes.
You'll see oil and hard wax finishes pushed online and on YouTube as "durable." They get that pitch so often precisely because durability is their weak spot. With normal everyday use, that thin protective layer wears through quickly. Once it does, the wood is unprotected and soaks up whatever lands on it:
- Plain water, which leaves the water rings everyone has seen on an old table
- Mineral stains from metal and iron
- Acidic spills like ketchup, coffee, wine or citrus
Any of those can leave a permanent mark.
What a Film Finish Won't Protect Against
Film finishes protect against almost everything, but not quite everything:
- Scratches. You can still scratch through a finish. How easily depends on how hard that particular finish is (more on that below).
- Heat. A hot pan or dish can soften or melt that thin layer of plastic. Once the finish is gone in that spot, the wood underneath is exposed again. Use trivets.
- Poor prep. A film finish that doesn't bond to the wood can peel. We cover how to prevent that in Part 3.
How Durable Is Each Finish? Two Industry Tests
Not all film finishes are equally tough. The coatings industry uses two common tests to measure it, and they look at different kinds of damage.
Pencil Hardness Test: Scratch Resistance
Pencils ranging from very soft (6B) to very hard (9H) are pushed across a cured finish, and the hardest pencil that doesn't scratch it sets the rating. It tells you how well a finish resists a single sharp scratch.
Here's how some common finishes stack up in published pencil hardness testing:
- Catalyzed polyurethane: 9H, the top of the scale
- Water-based polyurethane: 3H
- Low-VOC lacquer: 3H
- Clear shellac (aerosol): 3B, on the soft side of the scale
Taber Abrasion Test: Wear Over Time
The Taber abrasion test (ASTM D4060) measures something different: how well a finish holds up to repeated wear. A coated panel spins on a turntable under two abrasive wheels, and the result is reported as how many cycles it takes to wear through the finish, or how much finish is lost after a set number of cycles.
For tabletops, this is arguably the more telling test. A table rarely fails from one sharp scratch. It fails from thousands of plates, elbows and wipe-downs slowly wearing the finish thin. It also shows why oils and hard wax oils fail so quickly: there's barely any film there to wear through. If a manufacturer publishes Taber results on a product's technical data sheet, it's worth comparing.
Neither test tells the whole story on its own. Resistance to water, chemicals and heat matters too. But together they're a good way to compare finishes.
Top of the Ladder: 2K and Conversion Varnish
The most durable finishes out there are two-part, catalyzed finishes. You'll hear them called 2K (short for two-component) finishes, such as 2K polyurethane, or conversion varnish. The chemistry varies, but they work the same basic way, a lot like epoxy: you have a resin and a hardener, sometimes called a catalyst. Mix the catalyst into the finish, spray it on, and it dries fast and cures extremely hard.
These are industrial, commercial-grade finishes and they're top tier for durability. The tradeoffs: they're expensive, and they require professional spray equipment, since they dry too fast to brush or roll. (More on spraying in Part 4.)
Bottom of the Film Ladder: Shellac
Shellac is a perfect example of a film finish that isn't very durable. It's one of the oldest finishes around, it dries incredibly fast, and it's very easy to apply. But as the numbers show, it's soft and scratches easily. On top of that:
- Alcohol dissolves it. A spilled drink, perfume or hand sanitizer can soften or eat right through it.
- Water and heat leave marks. Shellac is known for cloudy white rings from wet glasses and warm dishes.
Shellac still has its place. It's a great sealer, a good barrier coat between an oil finish and a water-based topcoat, and a classic choice for drawer interiors and antique repairs. Just don't count on it as the top coat on a table that gets used every day.
Want to compare specific products? General Finishes publishes a topcoat comparison chart that rates the durability of each of their finishes side by side.
So Which Finish Should You Use?
I'm happy to use either type of finish. I just ask one question first: what is this piece going to be?
- Low contact, low wear: mantles, shelves, bookcases, wall pieces and decor. An oil or hard wax oil is a great choice here.
- Constant use: dining tables, kitchen islands, desks, anything people eat at, lean on or touch all day. Go with a durable film finish.
- Want both? You can put an oil finish under a film finish to get the color of oil with the protection of film. We explain how in Part 2.
The Takeaway
- Film finishes (poly, varnish, lacquer, epoxy) build a protective layer on top and are far more durable for everyday furniture.
- Penetrating finishes (oils, waxes, hard wax oils) bring out the most color but wear through quickly and leave the wood exposed.
- The pencil hardness test measures scratch resistance; the Taber abrasion test measures wear over time.
- 2K and conversion varnish are the toughest finishes; shellac is the softest film finish.
- Choose based on how the piece will be used.
Next up, Part 2: Oil Finishes and Hard Wax Oils: Beauty vs. Durability (and How to Get Both)
Starting a new project? Browse our live edge slabs and solid wood mantles.