What Happens if You Don’t Sand Between Coats of Polyurethane?

sanding table with polyurethane coat

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Polyurethane application may sound like a piece of cake, but if you don’t know what happens if you don’t sand between coats of polyurethane, it will be very frustrating! Luckily, we’ve gathered all the information you need before you begin sanding polyurethane or putting in an additional coat. 

Do You Really Have to Sand Between Coats of Polyurethane?

The primary reason for sanding between coats is to remove dust nibs and other imperfections. Applying the next coat after 24 hours is also supposed to help you create a stronger mechanical bond with the previous coat.

Even after sanding, fast-drying and most water-based polyurethanes will remain glossy, which won’t help with adhesion. Finally, sanding polyurethane between coats is required for a perfect finish.

Why Do Some Types of Polyurethane Need Sanding Between Coats While Others Don’t?

Time is a factor because of the need to sand between poly coatings. Fast-drying polyurethanes are more readily available now than they were many years ago because even after their initial coat or even last coat is applied, they will need some time for it to dry.

sanding table with polyurethane coat

Between coats of a fast-drying polyurethane finish, the wood surface need only be wiped down with a special type of cloth treated with a tacky material.

Not Sanding Between Polyurethane Coats

Many woodworkers say nothing serious will happen if you skip the process of sanding the next coat of polyurethane.

However, if it’s fine not to sand in the middle of each polyurethane coating process, why is this question being asked for so long? Why waste hours to complete doing something with no serious consequences? 

What Will Happen If I Don’t Sand in Between Coats of Polyurethane?

#1: Dust and Bubbles Will Get Trapped in Prior Coats

Every last bit of dust nibs and bubbles are removed from the poly before we call it a day. When applying polyurethane for the first time, bubbles are inevitable.

bubbles on polyurethane coat

Dust specks would appear if the bare wood wasn’t cleaned properly before the polyurethane finish was applied or if all the sanding dust was blown onto the wooden surfaces.

#2: Previous Polyurethane Coats Won’t Adhere Seamlessly

The surface can discolor or dry streaky if you don’t sand between coats of polyurethane. 

The formula inhibits the bonding of chemical solvents. And when its initial layer of paint has dried, a second can be applied to ensure everything stays.

Each successive coat of an oil-based polyurethane finish is applied on top of the previous one, though this fusion between only two coats does not always occur.

#3: It Will Be Impossible to Remove Brush Marks

Uneven brush marks are another common problem when you apply polyurethane to a wooden floor. That’s why it’s important to search out high-quality foam brushes especially if you want only one coat. Cleaning the brush you use for polyurethane application can also help to prevent uneven brush marks.

polyurethane varnish on wood

The initial and even next layers of polyurethane coatings may show brush strokes from certain products, but the last and final layer typically conceals these imperfections.

What Grit of Sandpaper to Use to Sand Polyurethane Between Coats?

Avoid scratching the surface by lightly sanding between coats of polyurethane. When you sand coats, this improves adhesion without affecting the previous coating.

For the majority of applications, 320 sandpaper grit will work just fine. However, you can use fine-grit sandpaper, such as 400 grit sandpaper, for a flawless result.

Can I Use an Orbital Sander Between Coats of Polyurethane?

Using woodworking tools such as an orbital sander with 220 grit sandpaper (or even steel wool) between coats is too abrasive and will not result in a nice finish.

person holding an orbital sander

Hand sanding with a flat block allows for greater control, resulting in a smoother, more durable finish.

Should You Sand The Final Coat Of Polyurethane?

Sanding the final coat of polyurethane will create scratches and a rough finish. To achieve a smooth finish, sand only between coats with very fine sandpaper, leaving the final coat unsanded.

How Many Coats of Polyurethane Should be Applied?

We recommend three coats to ensure a smooth finish. Two coats of paint will do if the furniture isn’t used often. Even in high-traffic areas, such as hardwood floors, you shouldn’t use more than three coats of the polyurethane finish.

Can You Apply Too Many Coats of Polyurethane?

In most cases, adding extra poly coats won’t ruin your finished product. However, sanding additional coats will take weeks, and you will need to spend more money; it will also not add much protection to the wood.

applying polyurethane varnish on hardwood floor

Can You Add Too Many Coats of Polyurethane to Your Floors?

There’s a three-coat maximum for floors and similar surfaces. Use a brush, a roller, or a wipe, whichever works best for you. For most projects, the next coat is all that’s needed to ensure protection for decades.

How Long Should You Wait Before Applying Each Coat of Polyurethane?

Each coat of polyurethane finish needs to dry before it can be sanded and applied. Water-based poly can be dry in as little as 6 hours, while oil-based poly can take up to 24 hours to cure.

How To Sand Polyurethane: 4 Easy Steps

Tools and Materials You’ll Need

Step #1: Prepare the Surface

Sweep up any surface debris and wash it with clean water. Scrub all possible imperfections on the surface with a cleaning detergent; such imperfections could include stains, grease, pencil marks, and so on. Allow the surface to dry after cleaning it.

wooden board and sandpaper

Step #2: Sand the Surface

Use papers with lower grits, beginning with a 100-grit and finishing with a finer paper at 150-grit. 220-grit is best for extra-fine surfaces and a smooth finish. The scuff sand method is not required.

Wipe the surface with a clean, lint-free rag moistened with a mineral spirit, then wipe with a tack cloth [1] to ensure it is dust-free.

Step #3: Stain (Optional)

If you use an oil-based poly, you should use an oil-based stain. If you use a water-based stain, use 220-grit sandpaper after it dries. Because some are stain and sanding sealer formulas, sealing the surface is unnecessary before you apply polyurethane multiple coats.

staining poplar wood

If your stain does not seal, you can use a polyurethane formula, but thin it with mineral spirits first.

Step #4: Apply Undiluted Polyurethane Coats

When working with water-based polyurethane, allow up to 6 hours for it to dry before sanding the surface slightly and applying polyurethane as the next layer.

If you’re using oil-based polyurethane, allow it to dry for up to 24 hours before sanding and applying the next and third coat. You can apply polyurethane to three thin layers on surfaces like the floor. 

Benefits of Polyurethane

Fills the Gap Between Rubber and Plastic

Since poly products are harder than rubber, they can withstand more loads. Furthermore, these formulas provide additional flexibility to what plastics have to offer. It is a popular choice for finishing high-traffic surfaces like the garage floor.

person applying polyurethane varnish on hardwood floor

Abrasion-Resistant

Polyurethanes outperform all other types of elastomers, metals, and plastics in terms of abrasion resistance. Its abrasion resistance is rated ten times higher than that of other materials.

Oil and Chemical Resistant

Most garage floors, kitchen tables, and other surfaces are finished with a polyurethane formula to avoid the risk of chemicals spilling. Polyurethane is used to coat such surfaces because of its high chemical resistance.

Highly Resilient

The hardness of a product is referred to as its resilience. If you’re looking for formulas that have a high level of resilience when applied to a surface, consider polyurethanes. A subsequent coat can withstand and absorb shock and hide all sanding dust.

sanding the top of a dresser

Types of Polyurethane Finishes

Sheen Type

High Gloss has a tough, flexible, shiny finish when dry. Semi-gloss polyurethane formula gives a sheen that is between gloss and flat, and satin gives a durable matte finish with both shine and a little extra color.

Method of Application

Another factor to consider is the application method. Many believe wipe-on is the best method for applying polyurethane varnish if you want a thin, clear coat on the wood floor.

Water-Based

Water-based poly does not last as long as oil-based polyurethane. Most people, however, prefer them because they are odorless and do not require a lot of ventilation; they also dry quickly, allowing you to complete your projects promptly. 

MINWAX Water-Based Oil-Modified Polyurethane

Oil-Based vs. Solvent-Based

Because volatile organic compounds are present in oil or solvent-based finishes, they must be applied in well-ventilated areas. These formulas are highly resistant to elements and other potential chemical spills. Due to its curing agents, the third coat dries to a clear finish when first applied but turns yellow with age.

What are the Alternatives if I Don’t Want to Sand Between Polyurethane Coats?

Option #1: Choose Other Products That Don’t Need Sanding

Some polymers can be used without ever touching sandpaper. Most commonly, these are polyurethanes that can be dissolved in water. You could also find high-quality oil-based varnishes wherein the first coat is enough.

Option #2: Don’t Use a Gloss Type of Finish

Oil-based polyurethanes consist of curing substances similar to satin finishes, so a semi-gloss finish is preferable over a satin polyurethane finish. When this happens, flaws become more noticeable, and subsequent coats don’t stick well unless sanded.

newly varnished table

Option #3: Use a One Coat Polyurethane

You can utilize an alternate strategy so you won’t need an additional coating. It’s oil-based, but it dries quickly, which is a plus.

However, one-coat poly is typically very thick and does not level well, making it a pain to work with. In addition, the poly’s rapid drying time means that brushstrokes, pencil marks, and dust nibs will stand out clearly.

FAQ

What happens if you don't sand between coats of Polycrylic?

If you don’t sand between coats of polycrylic, the first coat will be pestered with dust particles and grain, causing the topcoat to not cling to the base coat properly. Furthermore, you will notice uneven application instead of a smooth surface.

How do you smooth the final coat of polyurethane?

To smooth the final coat of polyurethane, sand lightly with the use of the dry sanding method, which involves only sandpaper, or the wet process, which involves mineral spirits or water and sandpaper.

Can I recoat polyurethane without sanding?

Refinishing wood coated with polyurethane necessitates sanding first. Use fine-grit sandpaper or no. 2 steel wool to sand the surface. Spray polyurethane in the same direction as the wood’s grain.

Should I sand between coats of Minwax polyurethane?

In between coats of Minwax Polyurethane, thorough sanding is recommended. Applying 320 grit sandpaper will help eliminate the fine dust that made its way to the wet surface.

Can you sand between coats of water-based polyurethane?

Sanding between water-based polyurethane layers is necessary to achieve a smooth and uniform finish. When every layer dries in 6 hours or less, even light sanding between multiple coats is unnecessary for inter-coat adhesion.

Do you have to sand between coats of spar urethane?

Spar urethane can be reapplied in 10-12 hours without sanding. After the first 24 hours, you need another sanding process using 220-320 grit paper. This will eliminate streaks and bubbles. Apply a thin coat of urethane polymers by brushing them on the wood grain.

Conclusion

Sanding multiple times and applying more coats may take a lot of time and money. But, if you skip this process, the polyurethane coat will not cling to the previous coat, and the last coat will eventually peel off, causing you to buy more and start the project again.

So, if you still don’t understand what happens if you don’t sand between coats of polyurethane, just know that you’ll end up spending even more if you skip the process!

Robert Johnson is a woodworker who takes joy in sharing his passion for creating to the rest of the world. His brainchild, Sawinery, allowed him to do so as well as connect with other craftsmen and women. He has since built an enviable workshop for himself and an equally impressive online accomplishment: an extensive resource site serving old timers and novices alike.
Robert Johnson

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