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A high-quality finishing brush, like the kind used to apply polyurethane, can cost close to $50. Brushes like this that are properly cleaned and well cared for can last up to 20 years. In this article, we’ll teach you how and why to clean and care for your brushes.
Cleaning a Polyurethane-coated Brush
Before starting, check your polyurethane can to determine if it’s water-based or oil-based. This process has three phases; dissolving the polyurethane, washing it from the bristles, and spin drying the brush.
Step One: Dissolve the Polyurethane
- Fill four plastic cups with a few inches of liquid. If you’re cleaning water-based polyurethane, use water. If your polyurethane is oil-based, choose mineral spirits, turpentine, or paint thinner.
- Insert the brush into the first cup. When you put a brush into a cup to soak or clean it, try to insert it at a 45 degree angle to minimize vertical stress on the bristles. With the bristles meeting the corner of the cup, gently press down on the brush handle, bending the bristles. Each time you bend the bristles, they separate slightly, allowing the solvent or water to penetrate into the core of the brush, dissolving the polyurethane.
- Flip the brush and repeat the process on the other side. Bend the bristles enough to let the solvent or water reach the ferrule, which is the metal part of the brush that holds the bristles together.
- Remove excess liquid. Lift the brush out of the liquid and press the sides of the bristles against the sides of the cup. If you’re using a tinted polyurethane, the liquid has likely gotten darker, perhaps becoming opaque. You’re ready to move on to the next cup as soon as the liquid you press out of the brush is lighter in color than the liquid in the cup. If you’re using clear polyurethane, aim for about 45 seconds of flipping and pressing.
- Repeat steps two, three, and four with the remaining cups. The liquid should get progressively lighter as you move from one cup to another. Four washes is usually enough to dissolve and dilute the polyurethane, but if it’s not, repeat the process. For ongoing projects, put lids on the four cups and label them with the numbers one through four. You can reuse the same mineral spirits to clean several brushes.
Step Two: Wash the Brush
- Soap the brush in the sink. Rinse the brush and hold it in your dominant hand. Put a generous line of dish soap in the opposite palm. Cup the bristle tips and move the handle back and forth, working the dish soap into the bristles. Use your fingers and thumb to massage the water and soap into the body of the brushhead.
- Rinse the soap out of the brush. Using the same technique, move your brush under the faucet and rinse out the water, soap, and mineral spirits. Repeat this step and the previous step at least four times, working the soap into the bristles of the brush and into the metal ferrule. If you’ve used a chemical solvent, note that they will prevent dish soap from lathering. As they are washed away, the amount of suds will significantly increase. This is a good way to check your progress. When the soap lathers normally, all the solvent has been removed.
- Comb out the solvent. Press the side of the bristles against the bottom of the sink. Use a nylon scrub brush to comb through the bristles of the brush. Start at the ferrule and comb in the same direction as the bristles, sweeping out any solvent remaining in the core of the brush. Thoroughly rinse the brush until you don’t see any soap suds in the sink.
Step Three: Spin and Dry
- Spin the brush. With the tips of the bristles pointing downward, hold the handle of the brush between your two palms. Rub your hands back and forth vigorously to eject excess water from the bristles.
- Reform the bristles. Run your hand along the brush head to encourage the bristles to return to their original positions. The bristles should be straight, not bent or twisted.
- Dry the brush. Hang the brush up to dry, or rest the ferrule on the side of the sink with the bristles over the basin. It takes about 24 hours for a brush to fully dry and be ready for another use. One of the best ways to store your brush is to hang it with the bristles pointing downward. Never rest a brush on the tips of its bristles. They aren’t strong enough to hold up the brush handle and will bend under its weight, damaging the brush and limiting it’s useful life.
Why Is It Important to Clean Polyurethane Off Brushes?
Some people advocate applying polyurethane with foam brushes, reasoning that they are cheap enough to simply throw away instead of cleaning. While foam brushes do a decent job of applying wood stain, they are not the best choice for applying a clear coat like polyurethane.
Foam brushes tend to leave ridges of finish to either side of each stroke. The air in the foam can be transferred into the polyurethane, causing bubbles.
Proper cleaning and maintenance will protect that investment, giving you a quality tool to use for years to come. You should thoroughly clean your brush every single time you use it. The entire process takes less than 15 minutes.
Beyond cleaning your brush, it is also important to clean polyurethane off of your hands. Even thought we advise wearing gloves, it is a good practice to get in the habit of.
To keep your brushes performing as well as they did the day you unwrapped them, thoroughly clean any brush you have used to apply polyurethane. Water-based polyurethane is dissolved with water, then washed from the bristles using dish soap. The same process is used for oil-based polyurethane, but chemical solvents are used instead of water.
This technique uses a series of four cups of solvent. If you have lids for the cups you use, the same solvent can be reused many times. When the brush is clean, spin out the excess and allow the brush to dry overnight.