Sanding Sealer vs Polyurethane

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Sanding sealer and polyurethane have some basic characteristics in common, but there are many differences as well. Find out when to use sanding sealer, when to use polyurethane, and which is better. We’ll compare and contrast these two products so you can choose the one that’s right for you.

What Is Sanding Sealer?

Sanding sealer is used to fill and even the pores of wood in order to prepare it for finishing with paint, stain, or clear coat. It is sold in a liquid form.

Different types of polyurethane

Sanding sealer is a lacquer that contains an active ingredient called zinc stearate. Zinc stearate, which has the consistency of soap, makes the lacquer fluffier, filling in wood pores quickly and building up a film on the surface of the wood. 

When the film is sanded away, the result is smooth, even, filled pores, and a surface that is ready to accept finish. 

Sanding sealer is sprayed or brushed onto wood, then allowed to dry for 30 to 60 minutes. Once dry, sand the surface of the wood as usual.

Why Is Sanding Sealer Used?

Sanding sealers are not necessary if your aim is a raw, natural, unpolished look or when you’re using softwood. They are only used to fill wood pores.

Wood pores are small, circular holes found only in hardwood. They are the remnants of the tree’s circulatory system.

When finish is applied to a hardwood with many deep pores, such as cedar, poplar, or spruce, it vanishes into these pores, requiring extra time and material. The result can often be blotchy or uneven. Some woods have wide, open pores that give them an uneven, pitted appearance.

Sanding sealer fills up the pores, smoothing the surface of the wood, and encourages even absorption of stain. They perform a similar function to grain fillers. Grain fillers are usually colored to match the wood, are smeared on with a putty knife, and are a pain to sand away.

Sanding sealer is clear and can be brushed on. It is softer than grain filler and easier to sand.

Sanding Sealer vs Shellac

Shellac, a mixture of resin and denatured alcohol, is often confused for sanding sealer.

Shellac is an effective sealer preventing smells from escaping or creating a barrier between oil and water finishes. Shellac is not nearly as durable as lacquer though.

Shellac also has nothing to do with sanding or filling pores.

What Is Polyurethane?

Polyurethane is a protective coating applied to wood. It can be applied directly to bare wood, or over a stain, paint, or other finish.

A man holding polyurethane can

Polyurethane is usually applied with a brush or a roller, although it can also be sprayed on. It is known for the excellent protection it provides from all kinds of damage.

Surfaces protected with polyurethane resist scratching, scraping, denting, moisture, fungus, mildew, and pests. It dries to a hard, clear, ‘water-white’ finish. Depending on the formulation, polyurethane may also be UV-resistant.

Polyurethane is best applied in thin, even coats. Proper application technique includes ‘tipping off’, or lightly brushing the surface to pop air bubbles. When applied too thickly, polyurethane takes on a plasticky appearance.

The first phase of polyurethane hardening happens when the solvent evaporates, leaving behind a solid surface that is dry to the touch.

Over the next 20 to 30 days, the polyurethane will continue to interact with the air around it, with individual molecules bonding more closely together. This process is called ‘curing’.

Polyurethane is frequently used to protect high-use surfaces like hardwood floors. It should be sanded between coats to promote good adhesion. Polyurethane is available in a variety of sheens, from fairly matte to high-gloss.

This popular and well-known varnish comes in two formulations: oil, or water.

Oil-based Polyurethane

Oil-based Polyurethane can take up to 24 hours to dry. It off-gasses significant amounts of VOCs, and has a pungent aroma that may linger for days or even weeks. Good ventilation is essential when applying oil-based polyurethane.

Water Polyurethane

Water polyurethane emits lower levels of VOCs and dries more quickly than oil-based versions. A coat of water-based polyurethane could be dry to the touch in as little as two hours.

Sanding Sealer vs Polyurethane

While sanding sealer and polyurethane are both products used in the wood finishing process, they are very different.

Similarities

The similarities between sanding sealer vs polyurethane are their application method, color, and consistency, as well as their VOC content and toxicity.

Color

Sanding sealer and polyurethane are both clear liquids.

Application Method

Both sanding sealer and polyurethane can be applied with a brush, roller, or spray device. Some polyurethanes can also be wiped on.

VOC content

Oil-based sanding sealers and polyurethanes release volatile organic compounds into the air, which have a negative effect on indoor air quality. If inhaled, health problems can occur.

Low-VOC versions of both sanding sealer and polyurethane are available. These formulas use acrylic and water bases instead of petroleum products, lowering the VOC emission.

Toxicity

A particularly worrisome substance called isocyanate is present in many polyurethanes and sanding sealers. Isocyanate is a respiratory toxin.

Differences

The differences between sanding sealer and polyurethane include the protection they provide and whether or not they fill wood pores.

Pore-Filling Action

Polyurethane will not fill the pores of wood. If you apply polyurethane to wood with unfilled pores, the finish will settle in the pores, revealing the uneven, pitted appearance. Sanding sealer is used to quickly fill in pores to reduce the time spent sanding without sacrificing a great result.

Protection

A major difference between polyurethane and sanding sealants is the amount of protection they provide. Or, in the case of sanding sealer, their lack of protection.

Polyurethane is one of the most protective coats you can give a coat of wood. When cured, this durable film can stand up to oil, grease, and water. It’s very difficult to ding, dent, chip, scuff, or scrape.

Sanding sealer doesn’t offer any protection to the wood. In fact, most of the sanding sealer should be removed by the time you’re finished. The only place sanding sealer should remain is in the pores of the wood.

Durability

Polyurethane is extremely durable. As a protective coating, you can expect it to last between five and ten years, depending on how it is used.

Sanding sealer is a very soft material and is not durable at all. In fact, problems can occur if too much sanding sealer remains on the wood when it is finished. The sanding sealer can slide, crack, or otherwise disconnect from the surface of the wood, disturbing the finish.

Major Differentiating Factor

The major differentiating factor between sanding sealer and polyurethane is when they are used in the finishing process.

Sanding sealer is a finishing base, used to prepare wood to accept a finish such as polyurethane.

Polyurethane is the final step in the finishing process and may be applied over sanding sealer.

When to Use Sanding Sealer

Use a sanding sealer to prepare porous wood for finish sanding. To use a sanding sealer for this purpose, brush it onto hardwood that has been sanded with 120 grit sandpaper.

Allow the sanding sealer to dry for 30 minutes. Sand down the sealer with 180 or 200 grit sandpaper to reveal a smooth, even finish. Wipe off any soapy, filmy excess with a clean rag.

Harwoods with deep or wide pores benefit most from sanding sealer, including mahogany, walnut, cedar, spruce, and poplar.

Once the sanding sealer has been mostly removed, finish with your choice of stain or a clear topcoat.

Learn what grit sandpaper to use if for primer before paint.

When to Use Polyurethane

Use polyurethane to protect wood from impact, moisture, and pest damage, or when you want a high-gloss clear coat over finished wood.

You can use polyurethane over any other finish, as long as it has completely cured first. To ease the transition, use a coat of shellac to prep the surface for polyurethane.

While water polyurethane can sometimes require four to six coats, oil-based poly seldom requires more than two.

Polyurethane is a film finish, meaning it sits on top of the surface of the wood rather than penetrating it. It is not as flexible as tung oil or spar varnish and is generally better suited for interior rather than exterior use.

Polyurethane can be used to protect anything made out of wood, from furniture to work surfaces. It is especially common to coat hardwood floors with polyurethane. These high-traffic areas require robust protection from scratches and scuffs.

Which Is Better, Sanding Sealer or Polyurethane?

Sanding sealer is a better choice if you are preparing wood to be finished and are struggling with the open pore structure. Quickly build up the surface with a sanding sealer, then sand it off to reveal perfectly smooth wood.

Polyurethane is best when you’re done sanding the wood and want to protect it. It can be applied over paint, stain, or bare wood.

You can even use sanding sealer and polyurethane together. Use sanding sealer to prepare the wood, and use polyurethane to form a clear protective coating. The result is smooth, even, elegant-looking wood.

Ellenkate grew up on job sites run by her family’s construction company. She earned her theater degree from The Hartt School, a prestigious performing arts conservatory in Connecticut. Her design and DIY work from her Chicago loft was featured in the Chicago Reader and on Apartment Therapy.