Last Updated on August 27, 2023 by Ernest Godia
Wood surfaces mainly comprise a closed face-grain (top and bottom) and an open end-grain (edges and contours). The closed face-grain isn’t as absorbent, but the open end-grain soaks up on the finish like a sponge. Ultimately, your wood surface is even on the face grain and splotchy along the end grain.
Sanding sealer and polyurethane are two popular finishes used to seal both types of wood grain, albeit with varying utility.
Should you go with a sanding sealer or polyurethane to achieve a smooth finish? This sanding sealer vs polyurethane article clarifies what a sanding sealer and polyurethane are and compares them side-by-side.
What Is Sanding Sealer?
A sanding sealer is a clear liquid used to seal wood pores, similar to other clear coats like polyurethane and shellac.
As the name suggests, it is sandable with minimal impact or damage to the wood. The outstanding characteristic of a sanding sealer is that it’s modified with zinc stearate or a resin (Vinyl).
Zinc stearate plays two significant roles:
- First, it gives the sanding sealer a much denser and thicker build, which means your surface gets more material to sand.
- Secondly, it has a lubricating effect and doesn’t drag or gum the sandpaper.
Wood fibers typically soak up liquids, and this can raise the grain and cause the wood to swell.
You must sand the surface each time you add another finish coat to get an even finish. Sanding sealer helps you avoid going through all that trouble.
Applying sanding sealer to your wood surface helps minimize rough texture and raised grain, creating a smooth finish.
Types of sanding sealer
Here are the types of sanding sealers available:
Lacquer-based sanding sealer
The lacquer-based sanding sealer has superior holdout qualities and requires less sanding. It is ideal for indoor surfaces like cabinets, panels, doors, and furniture. Apply the sealer before adding your coat of glossy or flat lacquer.
Vinyl sanding sealer
Finishes like polyurethane varnish, high-performance varnishes, and lacquers have poor adhesion to zinc stearate.
Vinyl sanding sealer is suitable in cases where finishes have an adverse reaction to zinc stearate. It also enhances bonding between different finishing products and promotes the curing of oil-based finishes on natural oil woods like rosewood and teak.
Dewaxed shellac sanding sealer
Use dewaxed shellac sanding sealer to achieve a smooth consistency with water-based polyurethane and finishes.
Applying dewaxed shellac over oil-based pores like rosewoods or aromatic red cedar blocks the compounds preventing adhesion with water-based finishes.
However, dewaxed shellac is less durable than lacquer or varnish sanding sealers, providing the wood with minimal resistance to heat and alcohol.
Glue-size sanding sealer
Glue size can work as a sanding sealer alternative as it strengthens and stiffens wood fibers, making them easy to sand. Glue size is concentrated, and to create a sanding sealer out of it, you’ll need to dilute it with water.
As a sanding sealer, glue size works perfectly on porous wood and medium-density fiberboard (MDF).
Sanding sealer pros and cons
Below are the pro and cons of using a sanding sealer:
- Flattens raised wood grain
- Quick-drying and easy to use
- Promotes adhesion between wood and the finish
- Prevents stain contamination and migration.
- Seals wood knots and pores for a smoother finish.
- Enables painting on any wood species
- It lacks moisture resistance qualities.
- Sanding sealer contains methoxymethyl ethoxy propanol, a toxic compound that can be absorbed via bare skin.
- Sanding sealer has fine dust that is airborne and can irritate the airways if inhaled.
- You can’t use it as a final finish.
What is Polyurethane?
Polyurethane is a plastic liquid that protects the wood against solvents, water, and harsh elements. The microscopic chains of resin molecules bond as it dries, making polyurethane a super-tough finish. You can apply polyurethane to floors, furniture, decks, and fences.
Types of polyurethane
There are two primary types of polyurethane, namely:
Water-based polyurethane has a white appearance but dries clear. It’s the better option when you want to preserve the natural wood color.
It is less toxic and contains lower levels of volatile organic compounds (VOCs), but you still need to wear your protective gear and work in a well-ventilated area when using it. It also cleans easily with soap and water.
Oil-based polyurethane dries with an amber tint, which can enhance the color of some woods.
It dries to form a durable protective coat on wood and other materials. However, it takes longer to dry and contains higher levels of VOCs than water-based polyurethane.
Oil-based poly offers great resistance to moisture, heat, and UV damage. And you must use mineral spirits or paint thinner to clean it.
Polyurethane pros and cons
Below are the advantages and drawbacks of polyurethane:
- It is highly durable
- It protects the wood against water, insect, fungi, and heat damage.
- You only need a few coats to achieve the desired protection with oil-based poly
- Oil-based polyurethane gives the wood a warm glow, and the water-based poly retains the wood’s natural beauty.
- Urethane products tend to gum and flake when sanded.
- It takes longer to dry and cure.
What is the difference between polyurethane and sanding sealer?
Polyurethane is primarily used as a protective and decorative topcoat, providing durability and various sheen options. Sanding sealer, on the other hand, acts as a preparatory coat to seal the wood and create a smooth surface before applying the final finish.
Sanding Sealer VS Polyurethane Detailed Comparison
Polyurethane and sanding sealer are commonly used finishes in woodworking, but they have different purposes and distinct characteristics. Here are their common uses and the differences between them
1. Polyurethane vs sanding sealer: Common uses
Polyurethane is commonly used as a protective finish for wood surfaces. It is a durable topcoat that protects the wood against moisture, stains, scratches, and general wear.
Thinned polyurethane can also be used as a pre-sealant or preliminary coat for highly porous woods.
Polyurethane offers versatility in application and finishes with oil-based or water-based formulations. It comes in different sheen levels, allowing you to choose your preferred level of shine.
Sanding sealer, on the other hand, is primarily used as a preliminary coat to prepare wood for the final finish. It seals wood pores, fills imperfections, and enhances adhesion for subsequent coats.
Sanding sealer has a thinner consistency, facilitating a smooth surface and leveling of the wood. It dries relatively quickly, usually within 15-30 minutes. This fast drying helps speed up the finishing process.
Sanding sealer is compatible with a variety of topcoats, allowing for flexibility in the final finish choice. Additionally, it creates a surface that is easier to sand than bare wood. This quality facilitates smoother results when preparing for subsequent coats or a final finish.
2. Sanding sealer vs polyurethane: Durability
Durability is a crucial aspect and parameter for testing a good quality product.
A sanding sealer will hold the topcoat for as long as possible. However, the number of coats you apply determines how long the topcoat will last. The more coats you use, the weaker the sanding sealer.
The zinc stearate in the sanding sealer makes finishes like lacquer soft and bristles. If you apply a thick coat of sanding sealer, your finish will eventually chip or crack.
When using a sanding sealer, don’t apply more than two coats; you must sand it down, which results in a less durable finish.
That isn’t the case with polyurethane. The latter (polyurethane) is a hard and durable coat over wood. The number of coats doesn’t influence its durability. Polyurethane takes the lead in this round.
3. Sanding sealer vs polyurethane: Versatility
Polyurethane is more versatile compared to sanding sealers.
You can only apply sanding sealer on bare wood. Its primary purpose is to prevent the finish from seeping into the wood grain. Sanding sealer doesn’t adhere to an existing finish, and as a matter of fact, it is an unnecessary step. Sanding sealer does bond well with stains, but you don’t need a coat of sanding sealer over stained wood.
Polyurethane, on the other hand, works on bare, stained, painted, and finished surfaces. It can work as a sealer and a durable topcoat.
4. Drying time of polyurethane vs sanding sealer for wood
The number of coats and their thickness influence the drying time of both sanding sealer and polyurethane.
The drying time of the sanding sealer depends on the type used. For instance, lacquer-based sanding sealer dries within an hour, while other types can take at least two hours.
If you want a quick-drying finish, polyurethane may not be ideal. Oil-based polyurethane can take approximately 12-24 hours to dry before you can sand and add subsequent coats. Water-based polyurethane dries within 2 hours of application.
The method and technique of application and environmental factors influence the drying time of poly.
Polyurethane dries much faster by wiping or spraying; the technique doesn’t compromise quality.
That said, the lacquer-based sanding sealer is the winner; water-based polyurethane follows closely.
5. Sanding sealer vs polyurethane: Bonding strength
When it comes to adhesion, polyurethane doesn’t play nice.
Sanding sealer works best on raw wood and isn’t compatible with other products. When working with lacquer-based sanding sealer, you can only apply lacquer over it, and the same goes for dewaxed shellac sanding sealer.
Manufacturers make it even harder to use a sanding sealer with other top coats by producing a sealer that only adheres to their products. That is an inconvenience if you already bought the sanding sealer, as you can’t apply any finish on it and, therefore, must revisit the hardware store.
Polyurethane forms strong bonds as the resin molecules in it dry and harden. Poly adheres perfectly to raw, stained, painted, or finished wood surfaces. You can also use it as a sealer and finish.
6. Sanding sealer vs polyurethane: Film level
The primary purpose of a sanding sealer is to ensure a uniform finish film level. Applying sanding sealer helps the wood grain soak the finish evenly without splotching.
Polyurethane doesn’t soak into the wood fiber unless you thin it to use as a preliminary coat. It simply sits on the surface. The base coat acts as a sealer, and the subsequent layers form a leveled film.
Polyurethane is self-leveling; you must give each layer ample drying time and lightly sand between coats.
Concerning the film level, it’s a tie.
7. How to apply sanding sealer vs polyurethane for wood
Sanding sealer is easy to apply; here is how to go about it:
- Get the wood as smooth as possible with a hand or orbital sander.
- Vacuum the sanding dust or wipe them away with a tack rag.
- Don’t shake the sanding sealer’s container, which creates air bubbles. Pour the portion you intend to use into a clean container and stir gently with a stir stick.
- Apply the sanding sealer with a synthetic brush. You can use the grain popping technique to wipe the sanded wood with a damp cloth to open up the wood pores. Open wood pores receive sanding sealer better.
- Let the first coat dry. Sand it down with fine-grit sandpaper and remove the dust.
- Apply your second coat and let it dry.
You might have trouble working with a water-based sanding sealer as it raises wood grain too much, and if you encounter such, switch to an oil-based sanding sealer.
Polyurethane is a tad bit challenging to apply with a brush or roller as it leaves brush strokes and roller lint on the surface. For better results, use a lint-free rag or sprayer. Here is how to apply polyurethane:
- Clean the surface to remove any dirt, dust, or grease.
- Sand the dry wood until smooth, and wipe the sanding dust with a tack rag.
- Apply the first coat of polyurethane; if you use a brush, thin the initial base coat with thinner or mineral spirits. With a sprayer, apply the poly in sweeping strokes and maintain a consistent motion and distance.
- You don’t need to sand between coats of water-based polyurethane. Let the first coat dry, and dust it. Remove the sanding dust with a damp rag or vacuum.
- Apply the second coat and let it dry.
On ease of application, the sanding sealer has a better head start as it is easy to apply with a brush, and you don’t risk getting brush strokes on your wood surface. Also, it dries much faster than polyurethane.
Should you sand the final coat of polyurethane?
No. Sanding the final coat of polyurethane is not always necessary or recommended, especially if you’re satisfied with the smoothness and appearance of the finish. Sanding the final coat of polyurethane lowers the level of gloss or shine. However, you can sand the final coat of poly to remove any visible brush marks, air bubbles, and dust nibs on the surface.
If you decide to sand the final coat of polyurethane, here are some general guidelines:
- Wait for the polyurethane to cure fully before sanding; poly can take several days to cure completely.
- Use fine-grit sandpaper, such as 320 grit or higher, to avoid removing too much of the finish. The goal is to create a smooth surface rather than sanding through the protective layer.
- Sand lightly and evenly across the entire surface to avoid creating noticeable irregularities.
- Tack away the sanding dust after finishing.
Sanding Sealer VS Polyurethane: FAQs
Below are answers to some of the frequently asked questions:
What is the difference between sanding sealer and varnish?
You can apply a coat of varnish or lacquer over the sanding sealer but not vice versa. When covering large surfaces, a coat of sanding sealer will speed up the process, but when covering smaller projects, the base coat of the varnish can work as a sealer.
Is polyurethane a wood sealer or a finish?
Polyurethane works both ways. As a sealer, you must thin the poly with mineral spirits or paint thinner, and as a finish, use either oil or water-based polyurethane to give your surface a glossy, even finish. Polyurethane finish protects surfaces from moisture, fungi, insects, and light damage. It is also durable.
Should I use sanding sealer before polyurethane?
Using sanding sealer before polyurethane is a matter of personal preference and the specific requirements of your project. A sanding sealer is particularly beneficial when working with open-grained woods, such as oak or mahogany, as it helps to fill the pores and create a smoother surface.
It can also be useful when applying a clear or light-colored polyurethane finish, as it reduces the risk of the wood’s natural tannins bleeding through and causing discoloration.
So it can be beneficial in certain situations, but it’s not mandatory for all woodworking projects.
Can I use sanding sealer as a finish?
Sanding sealer is a sealer, not a finish. Apply a sanding sealer to seal bare wood and act as a barrier between two incompatible finishes, like an oil-based stain and a water-based finish. You need to apply the sanding sealer, and after it dries, sand it, wipe the dust, and add a durable topcoat as a finish.
When do I need to use a sanding sealer?
You need to apply a sanding sealer when finishing wood with large pores like mahogany and walnut. Sanding sealer helps seal the pores and keeps the surface smooth and even. Also, with a coat of sanding sealer, you’ll need fewer layers of the finish.
Another instance best suited for sanding sealers is when trying to bond two products that naturally don’t merge.
Is sanding sealer necessary?
Sanding sealer is necessary if you want a consistent, smooth finish with less splotching. However, applying sanding sealer is optional. Consider using a sanding sealer to save time and material during the project.
Can you put polyurethane over the sanding sealer?
Yes, you can, but doing so is not a good idea. Sanding sealer mostly bonds well with shellac and lacquer. However, there are types of sanding sealers that go with polyurethane. Please read the manufacturer’s instructions on whether they recommend putting polyurethane over the sanding sealer.
Sanding Sealers VS Polyurethane: Final Thoughts
A sanding sealer isn’t a must-have on your projects, but having it is more beneficial than not. It doesn’t work as a finish, as its primary function is penetrating the wood pores and reducing grain rise. Also, it promotes adhesion between two incompatible wood finishes.
Polyurethane is both a sealer and a finish. Oil and water-based polyurethane protect surfaces against heat, light, and moisture damage. Polyurethane works as a wood sealant for outdoor surfaces. You’ll need to thin it out when using it as a sanding sealer alternative.
If you wish to minimize the project time, effort, and product you’ll use on wood, get a sanding sealer. And if you are looking for a sealer and finish combo, polyurethane is the way to go.