Polyurethane has become the most popular finish coating on the market, yet 90% of people applying polyurethane do it incorrectly.
Bubbling, running and puddling are the most frequent complaints of consumers applying polyurethane finishes I found while searching the internet. This can easily be avoided. I’ll explain here. As an option, you can watch my YouTube video “Bubble-Free Polyurethane Application Technique” where I visually demonstrate the correct steps to assuring a quality finish.
Sorry, I know this is lengthy but I think you’ll find it an especially relevant and a thorough step by step procedure for applying polyurethane properly. It can also serve as a review of my YouTube video on how to get the perfect finish with polyurethane.
For a NEW wood project – use 120 grit, 180 grit and 220 grit papers
For a re-finishing project – To get rid of some high spots, old paint or gouges I would add an 80 grit to the above three
A QUALITY finish brush – rated Above Average to Excellent
You’ll need a natural bristle brush for oil-based polyurethane application, or
A synthetic nylon bristle brush for water-based polyurethane application
A thinning agent
Mineral Spirits for oil-based polys, or
Water (I prefer distilled) for water-based polys
Tack Cloth – available at hardware stores, home centers, or online
Polyurethane – Available in three sheen’s Satin, Semi-Gloss, or Gloss
A suitable storage container – I prefer glass as some plastics will dissolve in mineral spirits. Nice to have one with a lid to seal for use on subsequent coats
A cleaning container for cleaning your brush after use
Patience and determination to get the job done right
SANDING BEFORE APPLYING POLYURETHANE
It starts with a good surface. Sandpaper is measured in grits with the lower the number the coarser the paper. Sanding tears out chucks of wood fibers. It actually creates scratches in the surface of the wood. The lower the grit, the deeper the scratching. Each step higher in grit lessens the scratching created by the previous lower grit sanding. Old wood or leveling a newly DIY project is the physical part. It requires the most effort and is, by far, the most time consuming.
Once the surface is level, usually done with an 80 grit paper, each step moving higher in grits gets quicker and quicker. You need to finish with a 180 grit minimum but I prefer to finish at 220 grit. So in summary the steps are 80 grit if necessary, 120 grit always, advancing to 180 grit always, and finishing with 220 grit preferred.
Before applying polyurethane make sure your surface is dust-free. Dust destroys projects! Your application environment should be free of most dust and your project should be free of all dust. This is important in achieving a smooth level finish. I use an air compressor, with a water filter, and then follow with a good TACK CLOTH WIPE-DOWN. The air compressor truly an option and not required for dust removal.
A word about air compressors: Air compressors are notorious for accumulating condensation. If your compressor expresses WET air it will actually drive water deep into the wood and may prevent the proper application of oil-based poly. I never want to intentionally wet raw wood when avoidable.
Forget cheapo applicators as they will ruin a good project. Foam brushes, sponges, rags, cheese cloth are all over the internet as suitable methods for applying polyurethane. As a result, so are complaints about polyurethane bubbles, just google polyurethane bubbles. Blog sites are replete with complaints and potential solutions. My theory, “do it right the first time to avoid looking for solutions”. Any serious woodworker will tell you that quality tools are a key to a quality outcome. Of course using the tools properly is also an important ingredient.
There’s a big difference between a paint brush and a finishing brush. Paint brush bristles are thin and stiff. Finish brushes are dense and soft. Paint brushes glob on thick material to hide surface below. Finish brushes delicately lay on a thin clear coat to accentuate the surface below. Heavy coats of paint levels as it dries (flattens out to hide brush marks and minor defects). Applying polyurethane (and most other finishes) in heavy coats creates bubbles, puddles and runs and becomes a defect in and of itself. Thick coats of finish also have a tendency to retain brush marks.
An above average finish brush is very dense and soft. It will run somewhere around $20 but with proper care will last years. An exceptional quality brush is extremely dense and feels like fine fur. It will run $40-$60 but will last a lifetime (with proper care). The brush in my YouTube video“Bubble-Free Polyurethane Application Technique”is 20+ years old and still in out-of-the-box condition.
TYPES OF BRUSHES
There are three basic types of brushes on the market.
Natural bristle brushes. Most natural bristle brushes are made from animal hair with ox hair being the most common. But be aware not all oxen possess the same texture of hair. Animal hair genetics, just like human genetics and hair texture, play a huge role in the quality of finishing brushes. Most regard European Ox hair as the gold standard for an ox hair brush.
For polyurethane I recommend either a pure China bristle brush (Chinese hog hair) or a combination of European ox hair and hog hair. Ox hair by itself is too soft and a lesser quality hog hair too stiff for a truly smooth finish.
Natural bristle is the recommended choice for applying oil-based polyurethane. These brushes are not recommended for water-based polyurethane because their bristles tend to absorb the polyurethane rather than transfer it to your surface.
Synthetic bristle brushes. For water-based polyurethane use a synthetic brush which is most frequently nylon. Nylon repels water. Just like natural bristle a quality synthetic brush will last a lifetime (with proper care). It should be dense and almost have the feel of natural hair.
In my YouTube video I show you how to soak your brush in mineral spirits or water before using it. The theory is that there is air trapped between the bristles of the brush and the ferrule (the metal retainer between the bristles and the handle). It also softens the bristles assuring a better application. Soaking and pressing the brush to the bottom of a container filled with solvent releases air (you’ll see air bubble out as you press lightly down).
Once done, GENTLY remove the brush and GENTLY drag it across old newspaper to dry it out. This process takes time but assures that there is no air trapped in the brush that can or will release on your project. This is an extra precaution step and something new I demonstrated in my video“Bubble-Free Polyurethane Application Technique” that adds an extra layer of security.
Polyurethane out of the can is extremely thick and hard to apply without incidents of bubbling, puddling or running. Manufacturers like to advertise ONE COAT RESULTS. They are hyping their product. The thicker the product is applied the more likely it will be flawed. Thicker surfaces provide more protection but thicker surfaces are best achieved by applying multiple thinner coats.
I thin my polyurethane by a ratio of 3-1. That’s three parts polyurethane to one part solvent. Solvents are mineral spirits for oil-based polyurethane and water for water-based polyurethane. Thinning reduces the thickness and makes it easy to apply. It also eliminates the propensity to bubble, run and puddle.
Polyurethane will separate in the can, or other storage container. GENTLY using a mixing stick stir the poly until you feel there are no lumps on the bottom or in the swirl. Once done GENTLY pour 3 parts poly into a separate container. Then GENTLY pour 1 part mineral spirits into the same container. GENTLY mix the two together.
Why is GENTLY capitalized? Think about a kid blowing bubbles through a straw both in water and then in milk, what happens to the bubbles? Air in polyurethane tends to get trapped and in the end creates bubbles on our surface.
POLYURETHANE BRUSHING TECHNIQUE
There is an old woodworkers adage that has stood the test of time and a staple of getting a good finish coat, it goes: “LAY IT ON THIN”. Remember finishing is about accentuating the surface below. At no time should a good finish job become apparent. In fact BAD FINISHES ARE SEEN and GOOD FINISHES GO UNNOTICED.
Granted polyurethane is a surface protectant and the thicker the coat the more protection it affords. But more protection is best achieved by additional thin coats not pouring on gobs of material. Laying it on thin is an important step in avoiding bubbling, puddling and running when applying finish coats, especially when applying polyurethane.
Let’s assume we are finishing a 20″x40″ vanity top and your brush is 3″ wide (the procedure is basically the same for all surfaces but this is the easiest to describe).
STEP 1 – LAYING IT DOWN
GENTLY dip your brush into the poly. Hold the brush over the container and let the overage drip off. Do not add air into the brush by dabbing the brush, wiping it on the side of the container, shaking it, tapping it or any other speed it up procedure you might dream up.
Mentally divide the length of the vanity top into length-wise strips of 2 ½ inches (2 ½” divided into 20″ wide = 8 long rows). Move the brush to your project a GENTLY set it down in the middle of the far back row. Without pressure slowly drag the brush to the right side edge (following the grain if possible). At this point you have the right 2 1/2″ – 3″ strip done. Notice that the middle of the 40″ strip where you first set your brush down has a little heavier puddle of poly.
Applying Polyurethane Correctly – The First Brush Stroke – After gently dipping in polyurethane
Now GENTLY lay your brush just to the right of the puddle and drag it across to the left side edge of that row. NOTE: We never used a back forth motion, a dabbing motion, a tapping motion or any other motion while loading the brush with poly or applying the poly to the surface. It’s all about avoiding the introduction of air, creating puddles and avoiding runs.
Viola . . .first row done.
Applying Polyurethane Correctly – The Second Brush Stroke – Do not dip in poly. Instead stretch out the puddle left in the middle.
Repeat for the second row. GENTLY dip your brush into the poly container, let drip (no dabs, taps, wipes, etc) and lay it on the next adjacent row in the middle, overlapping the previous row by ½” (or so). Now GENTLY and SLOWLY drag your brush to the right side edge.
Applying Polyurethane Correctly – The Third Brush Stroke – After gently dipping in poly. Be sure to overlap first and second strokes slightly.
Go back to the middle and just to the right of the middle puddle drag your brush across it and all the way to the left side edge. Viola again . . . second row done. Repeat overlapping each row approximately 1/2″ until you have the entire surface covered.
Step 2 – FEATHERING
Now here’s an important step. We are going to FEATHER OUT the applied poly. Do not dip your brush or apply more polyurethane, the application process is complete and it’s time to feather out what’s already laid down. Using the 8 rows start at the far back edge row, set your brush down at the extreme right side edge and GENTLY and SLOWLY drag your brush the entire 40″ from the right side edge to the left side edge. Once done with the first row, overlap ½” (or so) and repeat doing all rows from right to left.
Applying Polyurethane Correctly – The Feathering Strokes – Gently run your brush from one edge to the other until all eight rows are feathered.
Do not dip or add additional poly. Once done leave it alone. Avoid all temptations to overwork the coating.
This my friends is how we “LAY IT ON THIN”!!
THIN COATS OF POLYURETHANE ARE BETTER BUT THICK COATS OFFER MORE PROTECTION
Yep it’s true. Manufacturers advertise that they have a “ONE COAT” product. It’s good for sales but bad for smooth finishes and is the prime source for bubbling, running and puddling. Multiple thin coats are the best way to achieve the desired surface thickness.
So here’s where we apply additional coats. First coat, second coat, third coats they’re all the same procedure. Just keep repeating the above process until your get to the desired thickness. Here’s my COATING RECOMMENDATION based on typical usage.
Furniture that will not receive a lot of handling – 2 COATS
Counters and bar tops. Cabinets and wet surfaces – 3 COATS
Another manufacturer hype is no sanding between coats. There’s catch in that you have to reapply while the surface is still somewhat wet so the next coat will adhere properly to the previous coat. This is okay but has never been my personal preference. I always plan my project so the last thing I do for the day is apply the coating. Why? Because I like to let my project dry overnight so I can feel the surface, inspect for easily corrected flaws and see if the sheen is right for my intended look.
SANDING BETWEEN COATS OF POLYURETHANE
It’s true once the product has dried we must do a VERY LIGHT sanding to prepare the underlying surface for the next coat. LIGHT sanding with 220 grit sandpaper slightly scratches the surface which is necessary for proper adhesion between layers. Again I encourage you to watch my video“Bubble-Free Polyurethane Application Technique”to see how fast and simple the sanding actually is.
PS: Make sure you remove all dust. Tack cloth works but now that the wood has some protection you can use a dampened rag or paper towel as well.
CLEANING YOUR BRUSH
Here’s a bonus not in the video. Hopefully you’ve made a sensible investment in a good quality brush. Caring for it should be paramount. I clean my brushes in mineral spirits for oil-based products and water for water-based products.
My routine is pretty straight forward. I want all the poly (or paint, or varnish, or whatever) out of the bristles and out of the ferrule before quitting for the day. Step one is I soak and dab and mash and squeeze in the appropriate solvent (water or mineral spirits) not once, not twice, at least three or maybe more times in FRESH solvent until I see no more product leeching from my brush. I then go to the sink and wash thoroughly with dish soap and water using a scrub brush to comb though the bristles. Lastly I thoroughly rinse the brush, hand spin it and hang it to dry. Anal? Yep! Did I mention that the brush in my video is over 20 years old?