Why Sand Between Coats Of Polyurethane? – Detailed Answer

A hand painting the wood with a brush

Woodworking is a whole different world on its own. For some, it’s a task, a profession, and for others, it’s a unique form of art, a craft where creativity knows no limits. Whatever working with wood and crafting wooden pieces means to you, it has its own set of rules, dos, and don’ts. Whether you turn bowls, design furniture, do hard floors, make cabinets and countertops, or work on upholstery, leaving wood without a protective finish can cause it to dry quickly, snatch the life out of it, and make it look like nothing more than a rotten toast. Coating your wooden items with a finish or stain is mandatory. A finish or coating adds strength to wood and builds a protective barrier against harsh weather as well as other external elements. A highly recommend finish for this purpose is polyurethane. For better results, adding sand between polyurethane coats is highly recommend, but we’ll come to that in a bit. Let’s first understand polyurethane itself to learn about its application with sand better.

What Is Polyurethane

Polyurethane is a varnish or wood finish having clusters of resin molecules and is a top choice for most woodworkers. Polyurethane is widely admired for its high durability and super easy application and is known as one of the best finishes for wood protection. Polyurethane is available in two kinds based on the formula for which we have oil-based and water-based polyurethane available in the woodwork markets. Both kinds differ in terms of application and performance, each having its own pros and cons. Here are a few facts about both the kinds you’d want to acquaint yourself with.

Water-Based Polyurethane

  • It is a clear solution that appears milky in the can but dries into a crystalline, clear finish.
  • It has a thin consistency.
  • It’s the best choice if you wish to preserve the wood’s original color as the formula doesn’t react with the original pigments.
  • It dries in very little time. When you use it on hard floors, you can walk on them after waiting only for 10-12 hours.
  • You’d need more coats of this kind as the consistency is more watery than the oil-based variant.
  • Never ammonia-based cleaners on this finish. All you need is soap and water to clean surfaces finished by it.
  • It contains lesser VOCs compared to the oil-based variant yet needs proper ventilation during the application process.

Oil-Based Polyurethane

  • In contrast to the water-based variant, this polyurethane has a thicker consistency and takes longer to dry.
  • It helps improve the appearance of hard floors and gives them a glossy finish.
  • Due to its thick formula, you don’t need to apply many coats of this variant. Only fewer coats are enough to give a tough and durable film.
  • It costs lesser than water-based polyurethane.
  • It converts into a mild amber shade and thereby adds warmth to the wood’s original color.
  • To clean it, you need mineral spirits.
  • It contains more VOCs compared to the water-based finish, due to which it is not available everywhere in the country.

Types Of Polyurethane Finishes

Most of the polyurethane finishes don’t contain the intoxicating chemical urethane and are made from urethane polymers like polyesters and polyethers. Moreover, these products contain chemical solvents and curing agents. As for the finishes, polyurethane gives finished looks in three kinds of sheen:

High Or Full Gloss

As apparent by its name, this finish gives a high-definition shine and gloss once it dries. You can use this for both interior and exterior wood surfaces, giving them a solid, high-performing finish. This super glossy formula is ideal for heavy-duty areas like those receiving high volumes of foot traffic daily. The application of this finish is found high in industrial settings and used for covering manufactured metal such as iron and steel. It also helps cover other surfaces like concrete, walls, etc.


The semi-gloss polyurethane formula dries to finish, which lies between glossy and matte. This formula is widely used for wooden items like doors, floors, tables, kitchen tables, and furniture. The application is pretty straightforward and can be done using foam brushes, lamb-derived wool applicator, and bristle.

Satin Finish

Satin polyurethane gives a long-lasting matte finish with a hint of glaze and a more accentuated color. This finish mostly dries quickly.

Sanding Between Coats Of Polyurethane

There is no unanimous opinion from woodworkers on adding sand coats between polyurethane coats. Everyone sees it their way. By going through multiple opinions on the matter, we gathered that some woodworkers consider sanding an integral part of the job. In contrast, others feel it may be better to sand in between coats of polyurethane but not mandatory at all. They believe that polyurethane does the job well on its own, so sanding can be skipped if you’re short on time or unwilling to put in the extra effort.

According to most woodworkers, sanding helps scuff surfaces before staining and smoothens them out. When working with polyurethane, the underlying idea is to sand raw or new wood before using polyurethane to make a stable, leveled porosity and eliminate blemishes, pencil marks, and glue residues left after the carpenter’s job. Sand between coats of polyurethane helps level out dust nibs and leaves a flawless canvas. Adding scratches on the surface enables subsequent layers of polyurethane also to develop a mechanical bond. However, the bonding aspect may be an enhancement of the existent bonds that polyurethane offers alone. Another advantage you get by sanding is a strong adhesion between each coat. In other words, sanding enhances polyurethane’s adhesion properties.

The polyurethane locks itself into those scratches made by sanding. Although it’s practically not possible to make scratches in every bit of the surface, and there will be gaps- nonetheless, sanding does support bonding between the layers. Since polyurethane does take a good amount of time to dry, you’ll mostly find those dust nibs that you should sand out before putting another layer on it. It’s recommended that you give more attention to sanding thoroughly while using a glossy finish for the following two reasons:

  • Even the most minor flaws appear prominently in a gloss finish. In semi-gloss and satin finishes, these flaws disguise themselves well.
  • Since the flatting agents that give other finishes a dull and matte look are absent in glossy finishes. Because of that, other finishes don’t bond well with glossy surfaces.

So while you’re on the sanding job, choose a sandpaper grit that helps get rid of flaws effectively without leaving deeper scratches than you need. Recommended grits for this purpose include the 220 grit sandpaper for sanding before the first polyurethane coat, then regular “P” grade, the #400 or #300 grit for the layers. There is no need for coarser grits unless there are huge flaws and blemishes on the surface.

The correct method is to sand the wooden surface in the direction of the wood grain and not across it or in the opposite direction. Apart from the 220 grit sandpaper mentioned above, you may also use a belt sander to get the job done. Belt sanders help save energy and consume less time.

Don’t go too aggressive with sanding. A light hand at it should do the job. However, on the last grain, you will have to be more elaborate on sanding. A good job at sanding will reward you with a smooth and uniform surface by leveling the wood’s porosity.

Moreover, it’s better not to use a flat block for backing the sandpaper while sanding between coats to get rid of dust nibs. However, it may not be entirely wrong to do that when you have a truly flat surface. The biggest drawback of using a flat block is that you can easily end up clogging the sandpaper. The clogs will add wilder scratches to the finish that may peep through the next coats.

To Stain Or Not Stain?

If you wish to have a natural finish on your wooden surfaces, you don’t need to stain them before applying polyurethane. However, if you do stain your wooden structures for added coverage and a hint of color, never sand before the first polyurethane coat. If you sand the structure after staining it, it will scrape off the stain from random areas and create a very untidy, rough, and un-uniform look.

Skipping Sanding

Skipping on the sand between coats of polyurethane won’t make a lot of difference and impact how the finish turns out. However, they do feel sanding does the icing on the cake as it helps enhance adhesions between coats and renders a smooth and balanced finish.

It’s pretty obvious how an additional step in every process can eat up time. When you’re a bit too tight on the schedule, you can skip the sanding and proceed with the polyurethane coats without it. However, when you’re doing it without adding sand in between coats, experts have some tips to offer here. They recommend applying very thin polyurethane layers to the hard floors or any other surface and then generously allowing it time to dry before you add the next layer.

With Oil-Based Polyurethane

If you’re working with an oil-based polyurethane formula, you must wait for a minimum of 24 hours/one day after applying a coat of the product. Only after the previous coat has dried is when you can apply the next coats. The good news is that since the oil-based formula has a pretty thick consistency, you don’t need too many layers of this polyurethane variant. Experts recommend 2-3 coats of the oil-based formula and, you should be good to go.

With Water-Based Polyurethane

In case you’re working with water-based polyurethane, the drying time between each coat must be at least 6 hours. This means once you add a layer of this polyurethane, you must wait for a minimum of 6 hours or a little more for it to dry before you proceed to apply the next layer.

What If You Don’t Sand Between Coats Of Polyurethane?

After beholding the facts listed above, you must have arrived at a stance and answer to this question by now. Let us establish once and for all that nothing bad happens if you choose to skip sanding between coats of polyurethane. Your finish will turn out just as it should with no serious issues at all. However, sanding is a good, helpful step in the process and can be understood as a practice that rewards you with a bonus above the best you already get, as it ensures a flawless, leveled finish. In rare product combinations, sanding may be essential. For instance, you might have to do the sanding when you decide to use a water-based polyurethane over an oil-based stain, both of which aren’t a friendly match for each other. The sanding will be in the middle to ensure improved adhesion.

Other Frequently Asked Questions/Concerns Regarding Polyurethane

Here are some of the most frequently asked questions and raised concerns regarding the use of polyurethane:

Can you skip sanding while applying polyurethane on floors?

Whether you choose to put a coat of water-based polyurethane or one of the oil-based polyurethane, always make sure the one you choose is formulated for the kind of floors you have and product-surface compatibility. However, you can easily restore your wooden floors’ natural appeal and beauty without undergoing the mess, hassle, and cost of sanding off the previous finish to start from scratch.

What is the best way to smoothen out the last polyurethane coat?

You can give your final polyurethane surface a nice and smooth finish by sanding with 240-grit sandpaper lightly between each coat. Once you are done with the last coat, let it dry for a minimum of 24 hours. This is a practice recommended by experts and a standard to ensure an excellent finishing job. It’s fairly simple and nothing extraordinary. Furthermore, the hack is to sand bare wood before the process to achieve a smooth base.

Sanding Vs. Buffing- What’s the difference?

Sanding is a great technique to settle flaws and deep imperfections, allowing owners to change their hardwood floor’s colors.

As for buffing, it is great to ensure the longevity of wood boards. It’s pretty simple, quick, cost-friendly, and less disruptive.

If you decide to go sand between coats of polyurethane, what is the standard/basic way to do it?

Start with the bare wooden surface by sanding it nicely to achieve a smooth, flawless foundation for the coating process. Then start applying the polyurethane and complete the first coat. Here is the first time you sand a coat. The point to note here is that the first layer of polyurethane needs thorough sanding to turn out smooth. Also, don’t fret if the results don’t appear as flawless as you’d wanted. Then repeat the polyurethane coatings.

Make sure you let each coat dry completely. Then, to allow each subsequent polyurethane layer a surface to bond well with, gently sand between each coat using 320-grit sandpaper by wrapping it over a hard block.

How many coats of polyurethane are good for the job?

Typically, a minimum of two coats is recommended for protection. However, for surfaces that are likely to face moisture and hard usages like floors or any other, for that matter, you should give at least three coats for advanced protection. The finish becomes smooth with each coat.

Aren’t two coats of polyurethane enough?

Only two coats may just do enough to give your surface a little grip. But multiple coats are mostly better and aid polyurethane’s effects, making the coating stronger and adding a high level of luster.

How do you fix a mistake if one happens while using polyurethane?

It is common to make mistakes while using polyurethane. The good news is that many of them can be corrected by recoating and sanding. With the tip of a paintbrush, pop all the bubbles you see while spraying or painting polyurethane. If you see bubbles that have hardened into the finish, try flattening them using 220-grit sandpaper followed by a recoat. Moreover, keep an eye out to figure out why an error came up. Then take necessary precautions to avoid inconveniences repeatedly.

How many coats of polyurethane should be applied on hardwood floors?

Experts recommend at least two coats of polyurethane’s oil-based formula, followed by a coat of lacquer sealer, as the bare minimum for hardwood floors. In many cases (not all), extra coats mean extra protection. However, you must keep in mind that only one coat of the oil-based formula of polyurethane can be applied in a day, as it takes 24 hours to dry completely. Once the old coat is fully dry, only then can you apply the next coat.  

Is it good to keep layering up the polyurethane coats?

Not really. Three coats of polyurethane are enough, and beyond that, it won’t do much good. Furthermore, it’s neither needed nor suggested for any reason whatsoever. Every extra coat has to be buffed thoroughly, which essentially means that you’re almost buffing off the previous coat by a half. Polyurethane acts like a nail polish that takes longer to dry with every additional coat.

Is polyurethane a good clear coat for wood?

Yes! Polyurethane wood finishes are the best cleat coats. They are synthetic coats that offer high durability and resistance against water which qualifies them as the best clear coat for wood preservation.

How to renew hardwood floors?

You can get hardwood floors refinished and renewed in one day. Here is all you need to do:

  • Buy a hardwood floor kit available for refinishing.
  • Remove all the objects from that room clean the hardwood floor well.
  • Sand the floor with liquid.
  • Make the mop damp and wipe the floor.
  • Put the new finish on.

How to put polyurethane on both sides of a dual-sided wooden structure?

You can easily apply polyurethane to a dual-sided wooden structure like the two sides of a shelf. For polyurethane, we recommend using Minwax quick-dry oil-based polyurethane for a clear and semi-glossy look with a natural bristle brush. Apply four coats onto each side of the shelf. The point to note and ensure here is that the first and last coats need to be diluted using mineral spirits. Also, do the sanding between each coat using 220-grit sandpaper. Tap each sanded surface lightly to get rid of any dust.

Which polyurethane finish is the hardest to work with?

The moisture-cured urethane is seen as the hardest polyurethane finish in the market and has the highest VOC content. However, due to its super-fast drying, it’s a no-go option for beginners and DIY enthusiasts.

Is there a difference between resurfacing and refinishing wood floors?

Yes, there is a difference. Refinishing is about sanding off the topmost layer of the wood floor, followed by the application of fresh stain and lacquer over the same surface.

On the other hand, Resurfacing involves a more thorough process than merely adding a layer of polish on the uppermost surface.

How long does it take for polyurethane to cure?

Curing is when the surface treated with polyurethane undergoes a downtime to have the finish hardened, durable, with strongly adhered to the floor. The curing of polyurethane typically takes four weeks to complete or even more depending on the weather and other environmental conditions like humidity, high temperature, etc. The curing duration also depends on the number of polyurethane coats applied and how thickly each layer was applied.

The Bottom Line

Polyurethane is one of the most recommended resin-formula and clear coat finishes for wood, and both oil-based and water-based variants have their own merits and drawback. However, both work well with different wood surfaces. As far as sand between coats of polyurethane is concerned, it’s a recommended practice for smoothening out your wooden surfaces for a flawless application of polyurethane. However, if you wish to skip on the sanding bit, you always can without worrying about messing anything up. Sanding is good but becomes mandatory only in rare situations like correcting errors, likely to happen during polyurethane application.

For more details and assistance regarding polyurethane application and sanding, contact Period House Guru.

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