If you’re like me and you’ve made more than a handful of furniture projects, you probably have struggled with getting that flawless oil based polyurethane finish. Over the course of the last two years, I’ve built numerous projects in my home shop.
I’ve tried various wipe on poly and brush-on stains, urethanes, brush and spray-on polyurethane, and stains with polyurethane. Each with a various degree of results and many pieces I’ve had to redo because they were just awful.
By doing all these projects I have developed a technique to applying a brush-on polyurethane finish that provides me with excellent finished results.
So if you’ve struggled with how to get the brush marks out of polyurethane or not get them in the first place, this article is for you.
Spray on oil based polyurethane vs brush on
I am a home-based woodworker and do most of my work in my garage or basement. Most professional cabinet makers use urethane spray on finishes in spray booths with proper venting. Using a spray gun without having the proper resources will fill your home or workspace with explosive toxic vapors and an awful smell.
The same would also apply to spraying on polyurethane. I’ve also tried the brush-on urethane and couldn’t bear the smell of that either.
At this point, I’ve settled on three or four brush-on finishes that I use in my home based woodworking shop that I’ve mastered for the wood finish on the types of furniture projects I do.
What type of brush should I use?
Be sure to use quality paint brushes for oil based polyurethane projects. Use natural bristle brushes rather than a synthetic bristles brush. I’ve found the best brush to apply polyurethane is a Purdy brand and I am really happy with the results I achieve with them.
You might be wondering if you can use a foam brush. The answer is no because foam brushes quickly degrade when applying oil based poly. They start to sag and leave pieces of the brush in your work piece. Also, issues with bubbles. Some people do get good results with them when applying water-based polyurethane projects. But, this article exclusively covers oil-based polyurethane applications.
Applying an Oil-Based Polyurethane Coating
In this article, I’ll focus on my go-to oil-based polyurethane clear finish when I need something that looks good, is long lasting and durable, and easy to work with.
I’ve tried the satin oil-based poly finish, but found the clouding agent in it reduces the sheen, obscures or blurs any of the fine details in the wood grain. So whether you prefer a polyurethane satin finish vs. a semi gloss or gloss is a matter of preference.
Since I build desktops with spalted hardwoods such as maple or ash, low-luster sheens are not acceptable for me. I want a flawless brush-on polyurethane finish for my pieces that allows the subtle details of the wood to show through.
Usually, I purchase the Minwax semi-gloss from my local big box store. I know there are Minwax haters out there, but I suspect they had problems because they didn’t master the details and want to blame the product. General Finishes makes a good one too but is a bit pricier. If you’re a beginner or novice, stick with the Minwax until you start to master the process.
So here is what I do for a flawless brush-on polyurethane coating finish.
The first key is sanding. If it is a flat top that I joined together from multiple pieces of wood, there are usually some slight undulations in the joined planks, along with glue squeezed out. If this doesn’t apply to your project, go to Step Two or keep reading.
I know there are fans of hand planing tables or desk tops. I’m not a fan because of the work involved, and that is another basket of skills needed to tuning and using the planer.
Occasionally, I may use a power hand planer. For now, we’ll keep the process simple.
I’ve detailed all the equipment and techniques I use below to help you provide excellent results without all the trial and error. Also, take a look at our polyurethane quick start application guide “How To Apply Polyurethane Without Brush Marks” in PDF format.
Tools and Equipment
- Belt sander (optional) with 80 and 120 grit paper
- Random Orbital (RO) Sander with discs grits 80, 120, 180, 220
- Sandpaper sheets 120, 180, and 220 grits with sanding block
- Mineral spirits (spirits)
- Turpentine (turp)
- Glass jars-(2) for spirits and turpentine
- Latex gloves
- Seal Coat, Zinsser Bulls Eye 100% wax free shellac (I use clear)
- Semi-gloss or gloss oil-based brush on polyurethane. Minwax polyurethane works for me, but General Finishes makes a really good one too.
- Dust mask or respirator. Work outdoors if possible.
- Eye safety glasses
Step One: Belt Sanding
This step is optional. For really rough the lumber or for previously finished hardwood pieces when you want to make quick work of stripping old finish. I may start with a belt sander and 80 or 120 grit sandpaper until I bring it to level. If I started with 80 grit, I’ll sand up to the 120 grit to remove the 80 grit tracks.
Avoid belt sanding veneer surfaces or plywood as you’ll sand right through the thin layers.
After my first pass, especially anything that I used glue to assemble, I wipe the surface down with a paper towel dampened with mineral spirits. This removes any sanding dust, helps to reveal any missed glue shadows that you’ll need to remove, and gives you a clear picture of the grain or other problem areas.
When satisfied, go to Step Two. Sand evenly across the surface. Harder woods will typically take longer versus a softer wood.
Step Two: Random Orbital Sanding
My next step is sanding with a random orbital sander. Note: Using a RO sander is pretty simple, yet you may see that if not used properly, they will leave curly “C” swirl marks that you’ll have to sand out. Make sure you do not leave your sander in one spot and ensure that you power it on before placing it on wood.
If my last sanding was with the belt sander with the 120 grit, I’ll start there. I sand until I cannot remove any more visible marks or I’ve hit a point of diminishing returns.
I then sand up through the grits until I get the results I want. If you get to the 180 grit level and see some imperfections, back up a grit level or two and sand back up. To check your work, wipe with a paper towel dampened with mineral spirits to reveal and remove dust.
You are likely to still notice some imperfections. If you were to stop here and apply your finish to a lesser degree, this is what you’d likely see in the finished project. Your decision is to sand down a grit and back up to 180 or proceed to Step Three. If there are minor imperfections, you’ll likely be able to sand out in Step Three.
Step Three: Hand Sanding For A Smooth Finish
This is where the rubber meets the road and where we create a quality piece with manual elbow grease and a keen eye on the details to achieve that flawless brush-on polyurethane finish.
Starting with your 180 grit sandpaper and block, sand with the grain until you hit the point of diminishing returns with sanding marks from the RO sander. Go up to the 120 grit until you cannot remove any more marks from the previous sanding.
Wipe with mineral spirits to reveal any scratches or glue stains left behind.. Decide if you’ve achieved the results you want and sand again if needed and wipe again. Wiping with mineral spirits also helps the finish to lay down better.
Step Four: Apply Seal Coat Shellac
Brush on a liberal coat making sure you do not overbrush as it gets tacky and dries quickly. It should be dry in 20-30 minutes.
Give a quick pass with the 220 grit sandpaper to remove any grain raise and high spots from drips. This will give you a fairly smooth surface.
Wipe with a clean lint free cloth to remove any dust. If you’d like do a 2nd coat of the shellac, let it dry, sand and wipe off dust.
Clean your brush with the ammonia and warm water and let dry. I usually start with full strength.
Step Five: Apply Oil Based Polyurethane
At this point, you have completed the prep work and are now ready to apply the polyurethane. However, before you start thoroughly wipe down the piece to remove dust.
Note: The ideal application occurs at room temperature because it allows the polyurethane to naturally level itself. Working at temperatures other than room temperature will result in the polyurethane taking longer to dry.
You might be curious about why I recommend applying three coats. The reason is that each additional coat contributes to a more substantial build-up of the finish. Additionally, by sanding between each coat, you can eliminate brush marks and achieve a progressively smoother surface.
Oil Based Poly Dry Time
Be patient. Oil based poly usually takes 3-4 hours to dry before applying second and third coats. This could be slightly faster or slower depending on temperature and humidity.
Brush on a light first coat according to directions on the can and pay attention to dry times. If it’s cooler and damp, drying times will be longer.
When you are brushing, examine at an angle to see if you have missed any spots. You shouldn’t overbrush, but be aware that some spots may require a little more effort to break the surface tension for the finish to adhere.
Carefully feather out drip marks and build-ups at edges. If you miss any, sand out after the piece dries. The less you have from the first coat, the better your results and less work ahead.
Let dry until the surface is no longer tacky. When fully dry, examine for any drips or areas with little or no coverage and lightly sand.
Why sand between coats of polyurethane? You may notice brush marks, a sandy feel on the surface or a grainy feel in some areas. Sanding will help to minimize any brush marks, the grainy feel, or slight imperfections. Sand according to the following instructions:
Lightly hand sand with the 120 grit paper and sanding block. Sand with the grain being careful not to sand all the way through the finish. Sand enough to flatten any drips and smooth the gritty areas and to remove sheen.
Wipe the surface with a paper towel lightly dampened with mineral spirits to remove all sanding dust from this step. This is key to a smooth finish.
Clean brush with turpentine, wipe excess off brush with paper towel and make sure brush is clean and dry before using the brush for your next coat.
Brush on your second polyurethane coating, making sure you have covered all areas evenly, paying special attention to previous problem drip areas and feather to surface edges. Work fast, maintaining a wet edge, and examine the surface at different angles to see any problem missed areas before finishing this step.
Let it dry until no longer tacky. Examine surface for any phantom nibs or other potential defects. It should be silky smooth to touch. If you are satisfied with your results, you could stop here and clean your brush.
If you notice any problems, lightly hand sand the affected areas with the 220 grit sandpaper and sanding block.
Wipe the entire surface with spirits and apply a third coat. Clean brush when done
When applying oil based polyurethane I always apply a third coat. By the time I get here, I have a brush stroke free finish if I did everything else to a tee. It’s like they say, “The third time’s a charm.”
And there you have it, a solution for eliminating brush marks in polyurethane!