How to Clean Polyurethane From Brush

We know polyurethane, and we like polyurethane.  We use it as a finish for our woodworking projects often.  We have a bit of experience, then, in washing our brushes after using polyurethane, as well as cleaning up after its use.  

It’s important to remember that there are two kinds of polyurethane:  water-based polyurethane and oil-based polyurethane, and therefore it’s important to distinguish between them when it comes time to clean up.  The methods of cleanup are clearly dependent on that distinction, and we’ll cover that today.

First, some basics.

What Is Polyurethane?

We’ve often written about polyurethane in past pieces, and you can certainly refer to them for greater detail than we will provide here.

Basically, polyurethane is a blend of polymers and urethane.  Put those words together, poly- and urethane, to see where the name comes from: polyurethane.

Polymers are naturally occurring, almost plastics in the forms of silk and wool, and even as DNA and protein.  Polymers are also synthetic materials such as nylon and polyester (in our clothing, for instance), and even the Teflon that coats the pans we use in the kitchen.

Urethane isn’t really a plastic, but is a rubber of sorts, sort of in between the two, if you will.  

They are blended together with chemicals (that are not germane to today’s topic, so we won’t go any deeper into them), along with a medium to form the product we know as polyurethane.  As for that medium, there is both a water-based polyurethane and an oil-based polyurethane.  You are probably more familiar with the water-based poly, as are we.

When it is applied in its liquid form, polyurethane will dry to a hard plastic shell that will protect that workpiece from water and liquid spills.  It’s a film finish that doesn’t penetrate the wood as other finishes do (especially oil finishes like linseed oil) but rather sits on the surface of the wood to perform its protective services.

It applies in a clear form allowing either the natural beauty of the wood or the painted beauty to show through well.  When it has dried, it offers a durable top coat that resists both liquids and some heat.

Polyurethane is also used in a variety of other purposes and products, including:

• Glues
• Spandex
• Cushions for upholstered furniture, bedding, and car seats
• Medical tubings, including catheters
• Epoxy resins for the sealing of boat hulls

There is more, but this will give you an idea that polyurethane is more than a wood finish, although it is that, too.

The Two types of Polyurethane

To determine how best to clean polyurethane from your brush, it’s important to know and distinguish between the two types of polyurethane we mentioned.  Let’s look at each, and the cleaning method will become a bit more clear.

Water-based Polyurethane

Water is its medium for the polymers and urethane that make up poly rather than solvents.  It dries into that hard plastic shell on the wood’s surface, but you will likely apply more than a single coat to give the wood a greater measure of protection.  

As such, you’ll use more water-based polyurethane than the other type because more than a single coat will be necessary.  Then again, it is important to mention that these additional coats will make the workpiece that much more resistant to water, mold, and fungus.  That is the purpose of a top coat that is waterproof, hard, and durable.

Oil-Based Polyurethane

This, too, is a film top coat and will form a hard surface that provides the same degree of protection to the wood as does the water-based poly products.  

Oil-based polyurethane was once considered to be a more durable top coat wood finish than its water-based counterpart, but as a result of advancements in the manufacture of that water-based counterpart, most woodworkers consider them to be of equal durability today.

Those same woodworkers also consider the oil-based poly version to be very averse to abrasions and scratches.  For that reason, they will choose the oil-based polyurethane to be a better choice for use on floors, kitchen cabinets, and countertops, and other surfaces more visible and highly used.

It takes longer to dry than a water-based polyurethane (which can dry in as little as a couple of hours, although don’t apply more than 2 coats per day), perhaps up to 48 hours, and will take an additional 30 days to fully cure.  

Oil-based polyurethane is also toxic, and a respirator should be worn when in use.  It should also be used in a well-ventilated area, as its VOCs can also be harmful VOCs (volatile organic compounds) are emitted as gasses into the air and breathed in the absence of a respirator.  

Overexposure can result in some harm to the lungs.  Even with a respirator, VOCs can cause eye, nose, and throat irritation, so be sure to exercise a greater degree of caution when using an oil-based polyurethane.

They will also darken over time, which adds to the question why woodworkers would even use them – longer drying time, more susceptibility to denting, and that offensive odor.  Yet, it does have its uses and applications, we suppose, even with the durability advancements in water-based polyurethane products.

Applying Polyurethane

This is where we encounter differences that lead to decisions on cleaning methods.  We will not bother to mention wipe-on polyurethane products.  

Wipe-on polyurethane products are merely standard polys that have been thinned with mineral spirits.  The advantage to their use is no brush strokes or having to reach into difficult places.  A damp, lint-free cloth is all you need to apply it, wait the 2 hours it takes to dry, and apply again.

One word on safety:  be sure to let the cloth dry fully before you dispose of it in order to avoid combustion and fire as it dries and oxidation occurs.  A pile of cloths used either to wipe on or clear up after polyurethane use is dangerous and to be avoided.

Otherwise, we are looking at brush application of polyurethane, and the type you’ve chosen to work with will determine the cleanup method.

Cleaning a Water-Based Polyurethane From Your Brush

First, let’s talk about the brush.  A synthetic nylon brush is a wise choice for applying a water-based polyurethane.  It won’t pick up a lot of moisture while being used, and that will make a better and even coat on your workpiece.  They are also less expensive than a natural bristle brush.

Remember now that we’re applying a water-based polyurethane, so cleanup both after finishing and cleanup on the brush is simple and quick.  

We suggest wearing gloves for the cleaning.  Put some clean, warm water in a few buckets or clean cans, and dip the brush in one, then a second, and then another, bending the brushes back and forth with each submersion.

Then, off to the sink and rinse the brush under running warm water.  Use dish soap and give the brush a good wash and scrub with it.  Washing and rinsing it a couple of times won’t hurt and will ensure you have removed all poly from the bristles.

Hang and air-dry overnight, and it will be ready for use again the next day.  Yes, synthetic nylon brushes are less expensive than natural brushes, but money is money, and when you can reuse something, you should.  That’s it, too.  Nothing complicated about it.

Cleaning an Oil-Based Polyurethane From Your Brush

With an oil-based polyurethane, the cleaning process is a bit different.  Oil is the medium this time, not water.  The brush is going to be dirtier and in need of more care to clean it well.  You also have made an investment in a natural brush, and you want to keep using it.

Again, you will want to use gloves for this.  You will also need either turpentine, paint thinner, or mineral spirits for the first part of the cleaning.  The container your polyurethane came in will offer suggestions of which to use for the cleaning, and it’s always safe to follow those instructions.

Several containers or clean cans will be needed, to which you will want to add the chosen cleaning agent deep enough to reach the ferrule (the metal wrap that holds the bristles on the brush).  Dip the brush in the first one, bending the bristles to each side; let it drip a bit, and add to the next container, and so on down the line to loosen and remove the poly.  

You’ll know when that happens by the color of the cleaning agent – when it doesn’t change color anymore, you’ll know you’re done with that part of the process.  

Running warm water follows, and be sure to give the brush a really good rinse. Then reach for the dish soap and give the brush a good scrub with your hands and a nylon cleaning brush if one is handy. Make sure you get between the bristles thoroughly.

Rinse and repeat this step, as you are also removing the cleaning agent (turpentine, paint thinner, or mineral spirits), along with any lingering poly.  

You can pat it dry with a paper towel and then hang it for air drying, just as you did with the water-based poly brush.  The next day, it will be dry and ready for use again.

As a general rule, you will want to clean each brush while they are still wet from use.  If the poly dries even partially, it will be more difficult to clean for reuse, and you’ll be off to the hardware store to purchase another one.

It may surprise you to learn that you can get 20 – 30 years from a good brush, no matter what you use it for in your shop.  We found a video from a woodworker of 30+ years who has brushes he’s used for that period of time simply because he took care of them.

In this video, he talks about cleaning his brushes after using paint, polyurethane, varnish, and shellac, so you get a wide exposure to many of the things you use your brushes with on your projects.

Follow these simple steps, understanding the different cleaning needs for each of the types of polyurethane, and maybe your brushes will last for a couple of decades, too.

Last update on 2024-09-15 / Affiliate links / Images from Amazon Product Advertising API