Polyurethane Coats: How Many Are Needed?

Polyurethane is a clear coating that is often used to protect wooden surfaces such as floors, countertops, and tables. It is available in two types: oil-based and water-based. The number of coats of polyurethane required depends on the type of polyurethane, the specific project, and personal preferences. While one coat of polyurethane may be sufficient for some projects, multiple coats are generally recommended for proper protection and a smoother finish.

Oil-based polyurethane: pros, cons, and application

Oil-based polyurethane is a type of alkyd varnish that has been combined with polyurethane resin. Alkyd is a substance found in most varnishes and oil-based paints. The addition of polyurethane resin improves the alkyd varnish's durability, heat resistance, scratch resistance, and water resistance. Oil-based polyurethane varnishes are of thick consistency and are extremely durable when cured.

Pros of Oil-Based Polyurethane:

• Oil-based polyurethane is self-levelling, which means that it will become smooth when applied to a horizontal surface.
• Oil-based polyurethane cures to form a tough shell, so it's a good choice for applications where durability is a concern.
• Oil-based polyurethane coatings achieve a rich, golden glow in a few coats.
• Oil-based polyurethane is available in both spray and brush-on formats.
• Oil-based polyurethane creates a hard protective shell in fewer coatings than water-based polyurethane.
• Oil-based polyurethane is ideal for applications where you want a hard shell look without the numerous coats required with water-based polyurethane.
• Oil-based polyurethane is typically used for sealing butcher block countertops, refurbishing cabinets, finishing stair railings, and finishing wood floors.

Cons of Oil-Based Polyurethane:

• Oil-based polyurethane can be difficult to acquire due to the high level of volatile organic compounds (VOCs) it produces.
• Oil-based polyurethane takes a long time to dry, which means you will have to wait longer between coats.
• The varnish will leave an amber hue over the surface, which is especially apparent in lighter woods such as oak or birch, adding warmth to the colour of the wood.
• During application, the odour of oil-based polyurethane is sharp and pronounced, and many people find it objectionable.
• Oil-based polyurethane requires mineral spirits or paint thinner for cleanup.

Application of Oil-Based Polyurethane:

• Oil-based polyurethane can be applied by brush, wiping it on, or with a spray.
• The spray is the most common application technique, but those sensitive to fumes and VOCs should avoid using a spray polyurethane.
• Brush-on oil-based polyurethanes are ideal for flatter surfaces where you need a thick layer of durable polyurethane.
• You can use a wipe-on oil-based polyurethane for uneven, contoured surfaces such as crown mouldings and stair bannisters.

Number of Coats:

• For ideal results, you should use about three or four coats of oil-based polyurethane.
• You will need to sand the surface between each coat.
• Oil-based polyurethane is typically slower to dry and cure compared to water-based polyurethane.

Safety:

• Wear a NIOSH-approved P-95 respirator mask when applying oil-based polyurethane. The area should also be well-ventilated.
• Oil-based polyurethanes are made with highly flammable and toxic solvents, which are hazardous to your health.
• The cleanup process for oil-based polyurethanes requires exposure to high-odour hydrocarbon solvents, which can cause short- and long-term health problems.

Water-based polyurethane: pros, cons, and application

Water-based polyurethane is a combination of polyurethane and acrylic resin. It is thinner and less durable than oil-based polyurethane, requiring more coats to achieve a smooth finish. Water-based polyurethane is ideal for those who want a quick-drying, low-odour, and easy-to-clean finish. It is also a safer and more environmentally-friendly option, emitting a low number of VOCs.

Pros of Water-Based Polyurethane

• Water-based polyurethane dries faster than oil-based polyurethane, allowing for multiple coats in one day.
• It is safer and more environmentally-friendly, emitting a low number of VOCs.
• It has an insignificant odour and is safe for people to be around during application.
• It is easy to clean up with just soap and water.
• It is harder than oil-based polyurethane.
• It does not discolour over time, maintaining the natural colour of the wood.

Cons of Water-Based Polyurethane

• It requires more coats than oil-based polyurethane (four to eight coats are recommended).
• It is less durable and scratches more easily due to its hardness.
• It is more expensive than oil-based polyurethane.
• It can cause the grain of the wood to lift, requiring additional coats to smooth the surface.

Application of Water-Based Polyurethane

Water-based polyurethane can be applied with a brush, roller, spray, or rag. It is important to work in a well-ventilated area, even though the smell is not noxious. Due to its thin consistency, it can be challenging to see where it has been applied. Sanding between coats is not necessary but is recommended.

How to prepare the surface for coating

Preparing the surface for coating is a crucial step in achieving a smooth and durable finish when applying polyurethane. Here is a step-by-step guide on how to prepare the surface for coating:

Step 1: Clear the Surface

Before you begin sanding, ensure that the wooden surface is free of any obstructions or items that may hinder the sanding process. Remove any old paint, varnish, or other finishes that may be present on the wood. This step is important to ensure a smooth base for the polyurethane application.

Step 2: Sand the Surface

Sanding the surface of the wood is essential to create an even and smooth finish. Start with a coarse grit sandpaper, such as 80-grit or 100-grit, and gradually move to finer grits like 180-grit or 220-grit. Sand the entire surface thoroughly, removing any marks or scratches left by the previous grit. Finer grits help eliminate deeper scratches and create a smoother finish.

Step 3: Remove Dust and Residues

After sanding, it is crucial to remove any dust, dirt, or residue left on the surface. Use a vacuum with a soft brush attachment to remove loose dust particles. Then, wipe down the surface with a clean, lint-free cloth to eliminate any remaining oils or residues. This step ensures that the surface is clean and ready for the next steps.

Step 4: Raise the Grain (for Water-Based Polyurethane)

If you plan to use water-based polyurethane, it is recommended to wet the wooden surface before sanding to raise the grain. Lightly dampen the wood and then sand it with fine-grit sandpaper, such as 220-grit. This step helps to smooth out the surface and prevent the grain from lifting when applying the water-based polyurethane.

Step 5: Seal the Surface

Wood is a porous material, and without sealing, bubbles may form when applying polyurethane. To create a sealant, thin the polyurethane by mixing two parts of oil-based polyurethane with one part mineral spirits for oil-based products or dilute water-based polyurethane with water. Use a natural bristle brush to apply the sealant in long, even strokes, ensuring a smooth and consistent coat.

Step 6: Prepare the Surface for Adhesion (if Needed)

If you plan to apply a water-based polyurethane over an oil-based stain, additional preparation is necessary for proper adhesion. Use steel wool to roughen the surface slightly, creating a stronger bond between the stain and the polyurethane. This step ensures that the water-based polyurethane will adhere effectively to the underlying surface.

By following these steps, you will have a well-prepared surface ready for coating with polyurethane. Remember to work in a well-ventilated area and wear the appropriate protective gear, including a respiratory mask and goggles, during the preparation process.

How to apply the coats of polyurethane

Applying coats of polyurethane is a time-consuming process that requires patience and precision. Here is a detailed step-by-step guide on how to apply coats of polyurethane:

Preparation:

First, ensure you are working in a well-ventilated area to remove vapours and potential fumes. Use an exhaust fan facing out a window and open another window to let fresh air in. If you are sensitive to fumes, consider wearing a respirator with an organic cartridge.

Sanding:

Sand the wood with progressively finer grits of sandpaper to achieve a smooth, level surface. Start with medium sandpaper (100-grit), followed by fine paper (150-grit), and finish with extra-fine sanding using 220-grit paper. Remove dust with a shop vacuum and wipe down with a clean, lint-free cloth moistened with mineral spirits. Finally, use a tack cloth for the last dusting step.

Sealant:

Prepare the sealant by thinning the oil-based polyurethane with two parts polyurethane to one part mineral spirits. For water-based polyurethane, simply dilute with water. Stir gently with a flat stir stick; never shake the can to avoid introducing air bubbles. Apply the sealant with a natural-bristle brush using long, even strokes, catching any runs.

First Coat:

Wait 24 hours after applying the sealant before applying the first coat of polyurethane. Do not thin this coat as it will affect adhesion. Use a good-quality natural bristle brush, dipping it about 1 inch into the mixture. Overlap brush strokes for a smoother finish and catch any drips. Allow at least 12 hours for the surface to dry, then cut any drips with a razor blade, being careful not to cut below the surrounding surface.

Wet Sanding:

After the first coat has dried for 24 hours, wet sand the surface with 400-grit sandpaper mounted on a sanding block. Dip the sandpaper in water and use circular strokes to remove blemishes and dust bumps. Ensure you use enough water to lubricate the sandpaper and only sand deep enough to remove imperfections. Wipe the surface with a moist cloth, then dry it with a clean, dust-free cloth.

Second Coat:

Apply the second coat using the same procedure as the first, 24-48 hours after the first coat. If necessary, repeat the wet sanding and drying process for the second coat.

Polishing:

Wait at least 48 hours after the final coat, then polish the surface with an automotive rubbing compound. Dampen a clean cotton cloth with water and apply the compound in a circular motion. This will remove any scratches and restore the luster. Allow the surface to dry, then buff with a clean, dry cloth. If the finish is cloudy, apply more compound, let dry, and buff again until the desired shine is achieved.

Additional Tips:

• Always stir the polyurethane instead of shaking to prevent bubbles.
• For oil-based polyurethane, use a natural bristle brush to avoid air bubbles and brush marks.
• Thin the polyurethane for faster drying, but never thin the first coat as it affects adhesion.
• Keep your brush damp with a solvent (mineral spirits for oil-based, water for water-based) to aid in cleaning.
• Tap excess polyurethane off over the container instead of wiping on the rim to prevent bubbles.
• Always brush with the grain of the wood and work the polyurethane into the pores for an even finish.
• Apply thinner coats to minimise drips, and use a sharp razor to remove any runs.

Number of Coats:

The number of coats required depends on the type of polyurethane and the intended use of the surface. For oil-based polyurethane, three to four coats are recommended, while water-based polyurethane may need four to five coats due to its thinner consistency. Surfaces with heavier traffic will also require more coats.

How to finish the final coat

Finishing the final coat of polyurethane is a crucial step in achieving a smooth and flawless finish for your wooden surfaces. Here are some detailed instructions to guide you through the process:

Step 1: Sand the Previous Coat

Before applying the final coat, it is essential to sand the previous coat lightly with fine-grit sandpaper, such as 320-grit or 400-grit sandpaper. This process will help remove any imperfections, such as dust nibs or brush marks, and create a smoother surface for the final coat to adhere to. Ensure that you sand in the direction of the wood grain on the final pass. After sanding, wipe away the dust with a rag dipped in mineral spirits or a tack cloth to ensure a clean surface.

Step 2: Apply the Final Coat

When applying the final coat of polyurethane, use a high-quality brush, such as a natural bristle brush for oil-based polyurethane or a synthetic filament brush for water-based polyurethane. You can also use a foam brush or cloth, especially for water-based polyurethane, to avoid applying too much at once. Apply thin and even coats, ensuring that you follow the direction of the wood grain. If using a brush, overlap your brush strokes slightly and be mindful of any drips. For intricate or contoured surfaces, consider using a wipe-on application method to avoid drips and runs.

Step 3: Let the Final Coat Dry

Allow the final coat to dry thoroughly before proceeding further. The drying time will depend on the type of polyurethane and the environmental conditions, but it typically ranges from 24 to 48 hours. Ensure that the workspace is well-ventilated and maintain a consistent temperature of around 77 degrees Fahrenheit for optimal drying.

Step 4: Sand and Polish the Final Coat

Once the final coat is dry, you can enhance its smoothness and shine by sanding and polishing it. Use a very fine grit sandpaper, such as 600-grit or higher, and sand lightly in alternating directions to remove any remaining scratches. After sanding, wipe the surface with a damp cloth to remove any dust. Then, using a soft cotton cloth or polishing pad, apply a small amount of automotive paste wax or polishing compound and rub it in a circular motion. This will add an extra layer of protection and bring out the shine in your wooden surface.

Step 5: Final Touches

As a final step, you can use a brown paper bag to buff the surface, removing any remaining dust nibs and enhancing the smoothness of the finish. You can also apply a coat of wax to add protection and a silky-smooth feel to the surface. However, this step is optional and depends on your preference.

Remember, always work in a well-ventilated area, wear protective gear, and follow the specific instructions for the type of polyurethane you are using. With these steps, you'll be able to achieve a beautiful and durable finish on your wooden projects.

Frequently asked questions