Sanding Sealers | What They Are | How & When To Use Them
I meet a lot of people who are confused about what sanding sealers are, and what they’re used for. Is sanding sealer necessary to accomplish superior results in woodworking? Or is it just a luxury item that isn’t really necessary at all? Keep reading to find out everything you need to know, and probably more than you want to know, about sanding sealers, their uses, and pro tips about using them.
What Is a Sanding Sealer?
Sanding sealer is a translucent liquid-based wood finish. It’s applied to bare, naked wood to prep the surface for subsequent coatings of lacquer, shellac, paint, or another final coating.
The sealer improves the surface for finishing by closing off the pores in the wood, and helping to smooth out any grain lines. When the untreated wood is sealed in this manner, it allows the paint or other coating to be applied later to spread out uniformly, instead of being sucked into the wood.
After the dried and cured sanding sealer is sanded, the wood surface is better prepared to achieve a super-smooth, consistently colored top finish – faster and with less physical work involved.
Sanding is an art. Whether you use a sanding sealer or not, be gentle when you’re sanding. And whether you’re using a piece of sandpaper in your hand, sandpaper on a sanding block, an orbital sander, a belt sander, or any other type of sander, learn to appreciate the vibrations of the sander. Feel the sanding as it occurs. Stay in touch with the wood or other surface you’re working with on a mental level. Be patient with your work. Get into the art of sanding to achieve outstanding results every time!
What Is Sanding Sealer Made Of?
Polyurethane and shellac are transparent finish products that are popular for use as wood sealants. They penetrate into pores and micro-fissures in the wood, making it waterproof.
Sanding sealers are much like polyurethane and shellac in that they also penetrate into the pores and fissures to provide a waterproof finish. However, sending sealers contains something that polyurethane and shellac do not.
It’s called zinc stearate.
Zinc stearate is a derivative of stearic acid, which is a fluffy white hydrophobic (water-fearing) substance in its natural form. It has a soapy, slippery characteristic, and it is insoluble in water or alcohol. It can only be broken down with acid.
When zinc stearate is added to the liquid base of a sanding sealer, it does not chemically melt or otherwise change form. Rather, it stays as is, sinks to the bottom of the container, and never mixes with the liquid base. That’s why it’s important to thoroughly mix your sanding sealer before applying it!
When you apply a sanding sealer to bare wood, it provides a fuller, thicker buildup of material. In other words, it gives you something more to sand. Plus, because the zinc stearate has a soap-like, slippery texture, it makes sanding easier and faster.
You’ll notice that when you sand wood that has a sanding sealer applied to it, it will powder up quickly, much faster than wood that was treated with polyurethane, shellac, or another coating instead.
Further, the zinc stearate works to stop the sanding powder from coagulating on your sanding disc or sandpaper. The zinc stearate acts as a lubricant, helping to reduce sanding drag, while also increasing the pulverization of the sanding sealer into dust.
Did you know that shellac is derived from bug poop? Yep – excrement from the female lac bug (Kerria lacca).
It’s true! Shellac is a natural bioadhesive polymer found primarily on the trees in the forests of Thailand and India. After harvesting, it is processed into dry flakes, and then dissolved in alcohol to make the liquid we all know as shellac. Shellac is contained in numerous products – including woodworking products, cosmetics, and even for coating some foods and candies. Yuck!
When to Use Sanding Sealer
Sanding sealer is particularly effective for highly porous or large-grain woods – like oak, birch, and mahogany, for instance. Using a sanding sealer on porous wood will make clearcoat or a coat of paint cover much better with a superior-grade finish. However, sanding sealer is worthless, and in fact counterproductive, for achieving open-pore “raw” looking finishes that are quite popular today.
It’s true that sanding sealer closes off micro-fissures and pores in wood, much like polyurethane or shellac. However, sanding sealer is different from those other substances. It does not provide a strong finish coat, and it will not protect the surface from water damage, scratches, or UV light.
Smokey Ross from Advanced Construction & Remodeling in Cincinnati, Ohio explains, “Sanding sealers are available with water-based polyurethane bases and dewaxed shellac bases. Polyurethane-based sanding sealers are thinner than shellac-based sealers. Polyurethane-based sanding sealers dry faster and are better suited for high-end furniture work.”
Dewaxed shellac is an oil-based substance that will require paint thinner or another solvent for cleanup. Water-based sanding sealers are much easier to clean up after using, and you can do it with just soapy water.
Is Sanding Sealer Safe to Use?
While the zinc stearate in sanding sealers is non-toxic, some other ingredients are dangerous. You should always work with sanding sealers carefully, assuming their toxicity, and taking measures to limit your exposure. At a minimum, you need to ensure that you’re working in a well ventilated area.
Sanding sealers contain a chemical compound called methoxymethylethoxy propanol, which is highly toxic and can be absorbed through your skin and into your blood. So always wear a thick pair of latex gloves, and any other protective equipment you deem necessary.
Anytime you’re sanding anything, it’s important to consider your respiratory health. Even if you’re sanding raw wood, it’s not good to breathe the micro-particles that are created, and that will envelope you in the atmosphere.
Remember that the entire point behind sanding sealer is to create a full build-up that’s sanded easily. Sanding sealers create a microfine airborne dust that can wreak havoc in your breathing passages.
According to the professionals at Target Coatings, “Oil-based poly is made with highly flammable and toxic solvents to allow the urethane resin to level and form into a cured film. The clean-up process, too, requires exposure to high-odor hydrocarbon solvents now well documented to cause short- and long-term health problems.”
Comparatively, water-based polyurethane sanding sealers contain relatively few hazardous ingredients, at least according to OSHA. Water-based polys are known to have up to 50% less volatile organic compounds (VOCs) than their oil-based counterparts.
It’s also good to know that water-based sanding sealers cost more to manufacture than oil-based versions do. That means that you’ll typically pay more for a water-based sanding sealer, but remember, the additional expenditure can be well worth it when considering your long-term health.
The zinc stearate in sanding sealers creates a rather soft finish. Therefore, if you want to apply a hard final coat, like lacquer, over a soft coat, like a sanding sealer, the final code will be much more likely to crack and chip away. Because of that, shoot for a maximum of two coats of sanding sealer on any project. Further, make it a point to sand as much of the sanding sealer off of the product as possible.
Sanding Sealer Application Steps
Applying a sanding sealer isn’t difficult. You just need to move slowly, methodically, and stick to a good plan of action. Here are the basic steps involved with applying a sanding sealer:
- Take your time sanding the wood to be treated. Get the bare wood as smooth and flawless as possible before applying the sanding sealer.
- Vacuum the wood thoroughly to remove any loose dust, and then wipe it down with a clean cloth. Be meticulous and leave the surface as dust-free and perfect as possible.
- Thoroughly mix your sanding sealer, but don’t shake it! Shaking causes the product to bubble-up, and you won’t want bubbles in the mix. Instead, use a stirring stick to mix the zinc stearate gently but completely into the liquid base to achieve a uniform distribution.
- Use a brush or roller to apply a generous but not excessive coat of the sanding sealer to the surface of the wood. Make sure the lighting is good so that you can see any imperfections or anomalies. Take your time and enjoy the process.
- Become One with the Wood!
- Now, wait.
- Allow the sanding sealer to dry thoroughly before sanding it. Never rush your woodworking projects as the entire point is achieving the most beautiful and high-quality end product possible.
- Sand the first coat of sanding sealer uniformly and take as much off as you can without exposing the bare wood beneath. Again, take your time. A few extra minutes of love goes a long way towards achieving an impeccable end product. The devil’s in the details.
If needed, apply a second coat of sanding sealer in the same way as you did the first coat. Follow all application and sanding steps until your surface is excellently prepped for its final finishing.
When humidity, temperature, air flow, and other atmospheric conditions are all equal, you can count on an oil-based sealer to take about two to three times longer to dry in between coats relative to a water-based polyurethane,” Mark Roberts from Enterprise Builders in Hampton, New Jersey states. “I almost always choose a water-based sealer, even though they typically cost more. They save me time and effort – which is money!”
Sanding Sealer Summary
Sanding sealer is an excellent way to fill in large grains, fissures, knots, and other uneven wood surfaces. It waterproofs the pores in the wood so that subsequent finished coats don’t soak in excessively and diminish the uniformity of the end product.
However, you never have to apply a sanding sealer. A base coat of polyurethane, lacquer, shellac, or paint will also seal the pores and provide leveling effects for the wood’s grain. However, opting to use a sanding sealer will allow you to cut down on the time and energy spent on achieving the same result.
Sanding sealer is typically looked upon by most woodworkers as a luxury product to make their lives easier, not something that they must use in order to achieve the superior results desired.
Thank you for reading about sanding sealers, and I hope that you’ve gained a lot of useful information. Remember to always take your time when you’re sanding any kind of surface, be it wood, metal, drywall, or stone.
Sanding is an art. Sanding sealer can help you achieve a better product but you still have to move slowly – like the woodworking artist you are.
- What Is a Sanding Sealer? | Hunker
- What Is Sanding Sealer? (thespruce.com)
- What Is Sanding Sealer & When Is It Used? (fintechabrasives.com)
- Can You Use Sanding Sealer Over Stain? (& Vice Versa) – DIY Geeks
- (1) Woodworking Tips: Finishing – Why Use a Sanding Sealer – YouTube
- (1) Applying Sanding Sealer (Woodturning How-to) – YouTube
- (1) Chapter 4 – Sealers – Sanding Sealer – YouTube
- 5 ideal applications for sanding sealer | WOOD Magazine
- Zinc stearate – Wikipedia
- ZINC stearate | C36H70O4Zn – PubChem (nih.gov)
- What Is Shellac? Uses in the Beauty Industry and Environmental Concerns (treehugger.com)
- Kerria lacca – Wikipedia
- Water Based Polyurethane vs. Oil Based Polyurethane: Pros and Cons (targetcoatings.com)