Should You Sand After Polyurethane? [Inc. Between Coats]

Disclosure: We may get commissions for purchases made through links in this post.

Nothing beats polyurethane in woodwork finishes. But while doing your project, you are caught in a dilemma on whether to sand the final coat or not. And how about between coats? You are on the right page! We have consulted the experts and here is what they say.

You don’t need to sand the final coat of polyurethane unless the finish is noticeably uneven. However, it is highly important that you should sand polyurethane between coats to remove imperfections and for proper adhesion.

Sanding is a time-consuming and laborious step in polyurethane application. Despite this, it is your safest route to produce a smooth finish for your wood projects. Read on to learn why sanding is important, how it is effectively done, and how many polyurethane coats are needed for various projects!

Man applying polyurethane coating on the floor, Should You Sand After Polyurethane? [Inc. Between Coats]

Is Sanding Needed for the Final Coat of Polyurethane?

The final coat of polyurethane does not necessarily have to undergo sanding. Nothing happens if you don’t sand. Else, if you proceed to do so, it could be risky if it is not done properly. When this happens, sanding will be required and recoating the entire surface.

Worker applying polyurethane coating on the flooring

Also, there are instances when you were not able to achieve a smooth finish after your final application. Thus, sanding is your go-to option. But what if the problem seems deeper than it presents?

The best solution is to get into your polyurethane application correctly from the start of the process. You need to do the sanding between each coat.

Reasons Why Sanding Is Needed Between Coats of Polyurethane

Worker sanding the table

Applying polyurethane causes the grain of the wood to rise, thus creating blemishes and other unwanted marks on the surface. This can cause unevenness that requires sanding, among other reasons. Let's take a look at them in more detail.

To Remove Imperfections

When applying the first coat of polyurethane on bare wood, bubbles will likely appear. Some unsightly wood streaks will be visible too. Also, dust particles are inevitable, as you would normally expect when working with wood material.

Without sanding, these surface imperfections such as bubbles, streaks, and dust nibs may still appear on your finished wood surface. Overall, your project will look dirty and unbecoming and will remain that way no matter how you clean it.

These imperfections are a waste of the money, time, and energy you have invested in your wood project! While sanding takes a bit of elbow grease, it will help eliminate these problems.

The Succeeding Coats Will Adhere Properly

Polyurethane has a unique composition that’s slippery, thus preventing chemical solvents from bonding. During application, the topcoat simply settles above the coat underneath. It is expected that both coats will just simply combine with each other.

However, there are instances when the blending of two coats is not fool-proof. When this happens, the topcoat will eventually peel off in just a short time.

Sanding can help ensure a stronger adhesion during the blending of layers of polyurethane coats. It should seal off and tighten the chemical bonds, thus making it securely in place. Therefore, peeling is unlikely to happen.

To Remove Brush Marks

Man applying polyurethane on the table

Polyurethane is a thick and heavy substance, so it takes a few more minutes before it sets and fully levels out. Because of this, it cannot be avoided that paintbrush marks are visible on the layers of the coat during application. This holds true, especially on the first and second coatings.

On the third coating, brush marks can only be removed through sanding, and not by reapplying a thin coat as many woodworkers would have thought of.

To Prevent Cracking

Cracking is likely to happen when too much polyurethane is applied between coats or even on the finished surface. There is premature drying of the substance that could be due to the hot weather.

When cracking occurs, sanding is required before concealing the crack with another thin layer of polyurethane,

What Sandpaper Grit is Needed Between Polyurethane Coats?

Different textures and sizes of sanding paper

Grit refers to how abrasive your sandpaper is. Technically, it is determined by how many grains of abrasive particles per square inch it has. Thus, the coarser sandpapers have smaller grit while the fine grit sandpapers have higher grit numbers. The grade number or the grit value is usually written on the back of the sandpaper.

For reference, a 60 to 80 grit is coarse, 100 to 150 grit is medium, and 180 t0 220 grit is fine. A 320 or higher value is ultra-fine.

Check out this sandpaper kit on Amazon

Before you start to sand your polyurethane coat, be sure that the surface is completely cured or dried. Else, you risk yourself sanding off more of the coat than expected and might create further damage.

The objective of sanding between two coats is to strengthen the adherence but not to distort nor damage the original coat. Therefore, a light sanding is needed by using medium grit or #2 steel wool. Then remove the dust and clean the wood surface for remaining debris by wiping with a tack cloth.

Next, reapply polyurethane along the wood grain. As another layer of coat is added, the lighter the sanding should be since blemishes and scratches are expected to be less visible. Sandpaper grit 320 or a finer grit of 400, called clogging sandpaper, is used for a good and pristine finish.

With respect to woodworking tools, nothing can be compared to the comfort of manual sanding. However, for large projects or waiting time between coats become critical, then electric sander should come in handy.

Check out this electric sander on Amazon

How Many Coats of Polyurethane are Needed?

Worker applying polyurethane coating on the wooden planks

For water-based polyurethane, at least 5 coats and up to 8 coats are recommended for maximum protection. Top-quality brands can work with only 3 or 4 coats.

For oil-based polyurethane, at least 3 coats and up to 5 coats are recommended. The first coat is always buffed, so it loses the luster of the oil. The polyurethane gradually levels for every coat added. On the last coat, all the imperfections are gone and you will have an outstanding finish.

You might wonder if more coats than the required are better? Well, doing so won’t compromise your project. The downside is that it will not give you much better results. This means that you have more sanding to do and more time to spend on the whole project. All this translates to added costs without giving any more protection.

How Long Should You Wait Before Each Polyurethane Coat?

You need to wait for every polyurethane coat to completely dry before sanding and proceed to apply the next coat. The waiting time essentially varies depending on the manufacturer and whether it is water-based or oil-based.

Oil-based polyurethane takes longer to dry than its water-based counterpart. The former takes 12 to 24 hours while the latter can be dried between 6 to 12 hours.

Due to this time-consuming element, manufacturers have successfully marketed their fast-drying polyurethane. In this variant, you can recoat the oil-based type after 4 to 6 hours while the water-based can be recoated after 2 hours.

However, industry gurus express some caution, especially for newbies and DIYers. Though the waiting time may take quite long, it is still a better option than having to strip and redo your project. Also, fast-drying variants will not give you time to do corrections if there are any needed.

Of course, you would not want to be caught in a situation when you only realize that you have bubbles, streaks, dust nibs, lint, puddles, runs, or any other issues after you have finished the final coat. This means that you have to scrape off the entire surface. Much more, the product variant is more expensive, so it would be a lot to waste for a redo.

The slow approach doesn't look appealing, but definitely, you are on the safe side.

Actual Number of Coats for Different Applications

Man sanding his wooden table for a smoother finish

Sanding can be time-consuming and you'll need to leave some curing time before the next coat. Also, doing the sanding manually rather than with machines delays even further. Taken all together, this can add up quickly.

Due to these constraints, aside from the fast-drying variant, manufacturers have made many innovations such as modified water-oil based, less coat required, surface-specific, and other similar types.

The table below shows popular variants brands for common applications so you can have a rough idea of how many coats are needed with their curing time.

Variant

# of Coats Curing/Drying Time

Application

Wipe-On (oil-based) Polyurethane

3

2-3 hours

Carved surfaces with glossy or satin finish
Stain & Poly Combo

2

6 hours

Furniture, cabinets, trim
High-Build

2

4-6 hours

Tabletops
Fast Drying Polyurethane

2-3

4-6

Cabinets, floors, and furniture
Polyacrylic Poly

3

2

Light-colored wood
Water-based oil-modified

3

2

Doors, cabinets, and furniture
Oil-based spar urethane

3

4

Exterior doors and trim
Water-based spar urethane

4

2

Doors, trim, and furniture

Check out this fast-drying polyurethane on Amazon

Final Thoughts

Man applying polyurethane coating on the floor

Sanding the final coat of polyurethane is not necessary, but it is highly important to apply between coats to mark off imperfections and ensure proper adherence. With the correct procedure for sanding and sandpaper grit, you are assured of smooth and high-quality wood finishes for your projects.

Equally important is applying enough coats. Too much coating won’t do any good. Your next polyurethane coat is enough to give your project the protection it deserves for many years to come.

There are many polyurethane variants in the market today, each has its own specifications for the number of coats required, to address the consumer's need for faster and more efficient polyurethane application for various wood projects.

Want to know more about polyurethane? Check out these posts for further reading:

How to Get Polyurethane Off Hands [Oil-Based and Water-Based]

Forgot to Stir Polyurethane - Now What?

How to Apply Water-Based Polyurethane

Leave a Reply