Polyurethane

What is it?

Polyurethane is a family of plastics formed by reacting two ingredients (polyols and isocyanates) to create a hard, protective film, a flexible foam, or a strong adhesive. In home improvement, the term usually means a clear wood finish you brush, wipe, or spray onto surfaces. Once cured, it guards against scratches, moisture, and stains while adding sheen—from matte to high gloss—and, depending on the formula, either a warm amber tone or a nearly invisible clear coat.

Where DIYers use polyurethane

• Hardwood floors and stair treads
• Furniture (tables, chairs, dressers, bookcases)
• Cabinets and trim
• Interior doors and handrails
• Tabletops and bar tops
• Workbenches and shop fixtures
• Outdoor wood (with exterior “spar” urethane)

Beyond finishes, polyurethane also appears as construction adhesive and spray foam insulation, but this guide focuses on the clear finishing products.

Types and variations

• Oil-based polyurethane: Adds a warm amber color, levels out nicely, and builds a durable film with fewer coats. It has stronger odor, longer dry time, and will yellow further over time. Cleans up with mineral spirits.
• Water-based polyurethane: Dries fast, low odor, and stays much clearer with minimal yellowing. It usually needs more coats for similar build and can raise wood grain on the first coat. Cleans up with soap and water.
• Oil-modified water-based poly: A hybrid that applies and cleans up like water-based but delivers a hint of amber and better leveling. A popular choice for floors.
• Wipe-on polyurethane: A thinned formula (available in oil- and water-based) designed for applying with a cloth or pad. Great control and fewer brush marks; requires many thin coats.
• Spar urethane (exterior): Formulated to flex with temperature and humidity changes and resist UV. Use outdoors on doors, furniture, and trim. Not ideal for floors.
• Moisture-cured polyurethane: Extremely tough, solvent-heavy, and challenging to apply; mostly a pro-grade floor finish.
• Sheen levels: Matte, satin, semi-gloss, and gloss. Higher gloss shows more imperfections; satin and matte hide wear better.

Choosing the right polyurethane

• Desired look: For a warm, traditional tone on oak, cherry, or walnut, pick oil-based or oil-modified. For a light, natural look on maple, ash, or birch, choose water-based.
• Project location: Use interior formulas for indoor furniture and trim; choose spar urethane for exterior pieces.
• Dry time and odor: Need quick turnaround or working in a small space? Water-based is easier to live with.
• Durability needs: Floors and tabletops benefit from higher-build finishes. Oil-based or high-quality waterborne floor finishes are a good fit.
• Sheen: Satin is the most forgiving for everyday use; gloss highlights grain and depth but shows dust and scratches.
• Tools and comfort: If you’re new to brushing, consider wipe-on for small pieces or a water-based formula that levels quickly and dries fast.

How to apply polyurethane (step-by-step)

1. Prep the surface

   • Sand furniture to 180–220 grit (floors are typically sanded through 100–120, then screened to 220).
   • Remove all dust with a vacuum and a clean microfiber cloth. Avoid tack cloths on water-based finishes (they can leave residue).
   • For water-based poly on open-pore woods, “raise the grain” by wiping with a damp cloth, letting it dry, then sanding lightly with 220.

2. Check compatibility

   • If you stained the wood, let the stain fully dry per the label. Oil-based stains often need at least 24 hours.
   • When in doubt, apply a thin coat of dewaxed shellac as a barrier before water-based poly over an oil-based stain.

3. Stir, don’t shake

   • Stir gently to avoid bubbles and to evenly distribute flattening agents (the components that set sheen). Shaking introduces foam and bubbles.

4. Choose the right applicator

   • Oil-based: Natural-bristle brush or foam brush.
   • Water-based: High-quality synthetic-bristle brush or a foam pad.
   • Floors: Lambswool applicator or T-bar for large areas.

5. Apply thin, even coats

   • Work with the grain, maintain a wet edge, and don’t overbrush. Tip off lightly in one direction to reduce marks.
   • Let dry as directed: water-based often 2–4 hours, oil-based 8–24 hours (check the label and conditions).

6. Sand between coats

   • Lightly scuff with 220–320 grit or a fine sanding pad to remove dust nibs. Vacuum and wipe clean.
   • Plan on 2–3 coats for oil-based, 3–4 for water-based. Wipe-on may need 4–6+ coats.

7. Curing

   • “Dry to touch” is not fully cured. Avoid heavy use for several days. Full cure can take 7–30 days depending on product and conditions.

Safety tip: Lay oil-soaked rags flat to dry outdoors or store them submerged in water in a sealed metal container. Balled-up rags can self-heat and ignite.

Maintenance and care

• Cleaning: Use a soft cloth or mop with mild soap and water. Skip ammonia or abrasive cleaners.
• Protect: Add felt pads to chair feet, use coasters, and keep grit off floors with mats.
• Climate: Stable humidity limits wood movement and helps the finish last.
• Recoat timing: If the surface looks dull or worn but not down to bare wood, a light scuff and one or two new coats can refresh it.
• Spot repairs: For small scratches, sand lightly over the area and feather in fresh polyurethane. Matching sheen is key.

Common mistakes to avoid

• Applying thick coats that sag or trap bubbles—thin coats level better.
• Skipping surface prep or sanding between coats.
• Shaking the can, which introduces bubbles.
• Using steel wool under water-based poly (tiny steel particles can rust). Use synthetic pads instead.
• Applying interior poly outdoors or on an exterior door facing direct sun; use spar urethane.
• Coating over uncured stain or incompatible finishes without a barrier coat.
• Ignoring temperature and humidity guidelines; cold or humid conditions slow drying and can cause cloudiness.
• Returning used finish to the can, which can contaminate the whole batch.

Related terms

• Varnish: A broad category of film-forming finishes; many modern “varnishes” are polyurethane blends.
• Lacquer: Fast-drying solvent-based finish commonly sprayed; easy to repair but less chemical-resistant.
• Shellac: Alcohol-based, warm-toned sealer/finish; great as a barrier coat (dewaxed) and for small projects.
• Spar urethane: Exterior-rated polyurethane with UV inhibitors and flexibility.
• Danish oil/Tung oil: Penetrating finishes that leave a more natural feel but less film thickness.
• Epoxy: Thick, glassy, pour-on finish for bar tops; often topcoated with polyurethane for UV resistance.

Practical examples

• Dining table refresh: Sand to 220, remove dust, apply water-based polyurethane in satin for a clear, modern look. Three to four coats with light scuff sanding between coats.
• Oak floors update: After screening, apply an oil-modified water-based floor poly. It delivers a slight amber tone, fast recoat times, and lower odor, with three coats for durability.
• Exterior bench: Sand, then apply spar urethane. Brush on three thin coats, sanding lightly between. Expect periodic touch-ups each season for best protection.
• Handrail upgrade: Use wipe-on oil-based poly for control around spindles. Apply several thin coats for a smooth, hand-friendly surface.

With the right product and a careful, thin-coat approach, polyurethane delivers a durable finish that looks good and stands up to daily use across many home projects.