All About Polyurethane (2024)

Without some kind of finish to protect it from the elements, wood can become as dry and lifeless as day-old toast. Coating wood cabinetry, furniture, or trim with a clear finish, whether you stain it or not, gives it richness and depth while protecting it from knocks, scrapes, and the weather.

Use this guide to learn more about choosing and using polyurethane, the toughest of the clear coatings.

What Is Polyurethane?

It’s a kind of super-tough varnish formulated so that its microscopic chains of resin molecules will bond tightly with one another as it dries. The result is a finish that’s much more resistant to water, solvents, abrasion, and impacts than traditional varnishes.

Some polys have oils that give wood a warm, amber tone. If you want wood to keep its light color, use a water-based poly.

Oil, Water or Both?

Restrictions on VOC emissions have spurred the development of alternatives to oil-based polys. Consider the different characteristics of each type before you buy.

Oil-based: Turns slightly amber, which warms up a wood’s color. Forms a hard, durable film in a few coats. Dries more slowly than water-based polys, so there’s a longer wait between coats. Higher in VOCs than water-based finishes and therefore not available in all parts of the country. Cleans up with mineral spirits.
Water-based: Looks like milk in the can but dries crystal clear. Good if you’re trying to preserve a wood’s color. More watery than oil-based polys, so more coats are needed. Dries quickly. Lower in VOCs than oil-based finishes but still requires good ventilation during application. Cleans up with soap and water. Never use ammonia-based cleaners on the cured film.
Water-based oil-modified: Appears cloudy in the can but dries to a tough, amber-color film, like an oil. Fast drying and compatible with all woods; cleans up with water. Has the same low VOC levels as water-based polys.

Gloss, Semi-Gloss, or Satin?

Choose whichever sheen you like best; there’s no difference in durability. Just remember that the glossier the finish, the more it will show any underlying imperfections and any future wear and tear.

Can You Use Polyurethane Inside or Outside?

Most exterior polys can be used indoors, but interior polys should never be used outdoors; they lack the additives that protect exterior finishes from UV rays.

How to Apply Polyurethane: Brush On, Wipe On, or Spray?

Every poly has its preferred applicator, typically a brush or cloth. Some polys also come in aerosol spray cans.

Brush-on polys work best on flat surfaces where it’s important to build up a durable film. Brushes hold a lot of finish, so you can cover a wide area each time you load them up.
Wipe-on polys are best for contoured surfaces—crown molding and stair balusters—where brushing might create drips. Wipe-ons form thinner coats than brush-ons, so use them when wear isn’t a concern.
Spray-on polys come in handy on hard-to reach surfaces, such as shutter louvers and chair spindles. Aerosol sprays require good technique to avoid drips, and extra prep time to protect surfaces from overspray. Their thin films aren’t as tough as the ones you brush on.

Polyurethane: How Much Do You Need & How Long Will It Last?

12 months – The length of time a poly should be usable after you open the can. Label the lid with the date it was unsealed.
1 teaspoon – To keep a partially filled can of oil-based poly from skinning over, cover the finish with this amount of mineral spirits.
1 pint- The approximate amount of poly you’ll need to put three coats of finish on one chair.

Pick the Best Polyurethane

Each polyurethane is unique, thanks to tweaks in its chemical makeup; choose the one that best matches your particular project.

Interior: Wipe-On

Good for: Carved, embossed, or profiled surfaces where a brush could leave drips. Also useful for hiding superficial scratches in previously finished wood. Its thin film offers moderate protection from abrasion. Available in gloss and satin sheens, and in oil-based and water-based oil-modified formulations.
Apply with: lint-free cloth
Number of coats: three
Hours between coats: 2 to 3
VOCs: 580 grams per liter for oil-based; 275 grams per liter for water-based oil-modified

Interior: Stain-and-Poly Combo (oil-based)

Interior: High-build (oil-based)

Good for: Tabletops and other surfaces subject to abrasion. Provides maximum durability with just two coats. Apply thin coats to prevent drips and wrinkles from forming. Available in gloss, semi-gloss, and satin.
Apply with: natural-bristle brush
Number of coats: two
Hours between coats: 4 to 6
VOCs: 350 grams per liter

Interior: Fast-drying (oil-based)

Good for: Cabinets, floors, furniture, and trim such as wainscot, where abrasion resistance and durability are important. This versatile all-around poly provides good protection on a variety of wood surfaces. Available in gloss, semi-gloss, and satin.
Apply with: natural-bristle brush or a can of aerosol spray
Number of coats: two to three
Hours between coats: 4 to 6
VOCs: 450 grams per liter

Interior: Water-based

Good for: Light-colored woods and stains where ambering would be undesirable. Blended with acrylic resins, it goes on milky but quickly dries crystal clear. Not as durable as oil-based polys. Available in gloss, semi-gloss, and satin.
Apply with: synthetic-filament brush or a can of aerosol spray
Number of coats: three
Hours between coats: 2
VOCs: 275 grams per liter

Interior: Water-based Oil-modified

Good for: Doors, cabinets, furniture and floors. Combines the durability and ambering of an oil with the fast drying time, low VOC content, and easy cleanup of a water-based product. Available in gloss, semi-gloss, and satin.
Apply with: synthetic-filament brush or a can of aerosol spray
Number of coats: three
Hours between coats: 2
VOCs: 275 grams per liter

Exterior: Spar Urethane

Good for: Exterior doors, trim, and furniture. Contains UV absorbers that protect the finish and the wood from the sun’s rays. Made with a special blend of oils and resins that allows it to flex as the wood surface expands and contracts. Recoat the finish when it turns dull, typically once a year. Available in gloss, semi-gloss, and satin, and in oil-based and water-based formulas.
Oil-based spar urethane
Apply with: synthetic-filament brush or a can of aerosol spray
Number of coats: three
Hours between coats: 4
VOCs: 450 grams per liter

Water-based spar urethane

Apply with: synthetic-filament brush or a can of aerosol spray
Number of coats: four
Hours between coats: 2
VOCs: 275 grams per liter

Polyurethane Dos & Don’ts

In addition to reading the directions on the can, keep these basics in mind.

Do…

Test the old finish

Acetone-based nail-polish remover softens lacquer. Water drops turn wax white in about 10 minutes. If either lacquer or wax are present, strip them before proceeding.

Stir the can

Do this before and during application to evenly blend the ingredients that control sheen and UV resistance.

Apply thin coats. Thick ones take longer to dry and are more likely to drip or wrinkle.

Sand between coats.

Using 220-grit paper helps ensure good adhesion and smooths away imperfections.

Wipe after sanding.

Dust interferes with adhesion and leaves unsightly bumps in the finish. Use a cloth dampened with mineral spirits on oil-based finishes; use a water-dampened cloth on water-based finishes. A dry microfiber cloth also works well with both finishes.

Polyurethane Dos & Don’ts

Don’t…

Shake the can.

Shaking creates bubbles, which leave a rough, pitted surface. It’s okay to shake wipe-on polys, however.

Thin the first coat.

Thinning doesn’t improve adhesion, and you’ll have to apply more coats and use more solvent.

Use tack cloths.

Some contain chemicals that prevent good adhesion.

Wax the finish.

Wax isn’t durable and will interfere with any subsequent coats.

Polyurethane Application Tips

How to Apply Polyurethane: When working with any finish

Choose the right brush

Use natural bristle for oil-based finishes and synthetic filament (polyester, nylon, or a blend of the two) for water-based and water-based oil-modified finishes. Don’t use rollers or foam brushes—they create bubbles.

Dampen the brush with a solvent

Your brush will be easier to clean, and will go longer between cleanings, if its bristles are dipped first in mineral spirits, if you’re using an oil-based poly, or in water, if the finish is water-based. Before using the brush, rapidly roll the handle between your hands inside a cardboard box to eliminate excess.

Pour some finish into a clean container

Working from a separate container prevents the dust and other particles picked up by the brush from contaminating the finish in the can.

Tap, don’t wipe

After dipping about one-third of the brush into the finish, gently slap it against the inside of the container. Scraping the bristles over the edge of a container leaves the brush too dry to apply a coat of the right thickness.

Always brush or wipe with the grain

It’s the best way to work the finish into the wood pores and ensure an even appearance.

When working with water-based finishes

Dampen the wood first

Wipe the wood with a damp cloth to raise the fibers; after the surface dries, knock them back with 220-grit sandpaper.

Apply and move on

Too much brushing can leave these fast-drying polys rough and ragged.

Don’t smooth with steel wool

It leaves behind tiny bits of steel that you can’t see—until they rust. Use sandpaper instead.

Polyurethane is a robust varnish designed to protect and enhance wooden surfaces. It forms a durable finish by bonding resin molecules, providing resistance to water, solvents, abrasion, and impacts. The choice between oil-based, water-based, or water-based oil-modified polyurethanes depends on factors like color preference, drying time, VOC levels, and application ease.

Oil-based Polyurethane: Offers a warm amber tone to wood, dries slowly, and forms a durable film but has higher VOCs, necessitating mineral spirit cleanup.

Water-based Polyurethane: Dries crystal clear, ideal for preserving a wood's light color, dries quickly, and has lower VOCs, cleaned up with soap and water.

Water-based Oil-modified Polyurethane: Combines the durability of oil with fast drying, low VOCs, and easy cleanup with water.

Gloss, Semi-Gloss, or Satin Finish: The sheen choice doesn't affect durability but impacts how imperfections and wear appear.

Indoor vs. Outdoor Use: Exterior polyurethanes protect against UV rays and weather elements, while interior versions lack UV protection and shouldn't be used outdoors.

Application Methods: Brush-on for flat surfaces, wipe-on for contoured areas, and spray for hard-to-reach spots. Each method has its advantages in coverage and durability.

Dos & Don’ts: Do test the old finish, stir the can thoroughly, apply thin coats, sand between coats, and wipe after sanding. Don't shake the can, thin the first coat, use tack cloths, or wax the finish.

Application Tips: Choose the right brush based on the finish type, dampen the brush with an appropriate solvent, pour finish into a separate container, tap the brush to remove excess finish, apply with the grain, and be cautious with water-based finishes, dampening the wood before application and avoiding over-brushing.

Polyurethane comes in various formulations tailored for specific applications, from wipe-on for carved surfaces to high-build for tabletops. Understanding the differences and best practices for application ensures a successful and durable finish on your wooden projects.