Polyurethane 101
How to Apply polyurethane
Why Polyurethane
Polyurethane is a durable and easy to work with protective wood finish. It is a polymer that is actually a form of plastic that seals the wood and protects it.
Polyurethane can be found in both oil-based and water-based formulas. The majority of my projects use oil-based polyurethane because I prefer the slightly warmer tone that it provides.
There are also spray-application polyurethanes, but I tend to avoid them. Polyurethane is most commonly applied with a brush but a rag can work as well. A minimum of two (2) coats should be applied but I recommend at least three (3) coats for most projects. We will get into dry times a bit further down.
Choosing your base (water based vs oil based):
As mentioned above, I tend to use oil-based polyurethane, it pulls the natural beauty from the wood and offers a deep warm (but not tacky) glow. Oil-based polyurethane typically requires fewer coats and is a bit thicker. The downside is that oil-based polyurethane will take longer to dry (8 hours between coats for Oil vs. 2 hours between coats for water) – this leaves your project susceptible to dirt, debris and dust. It also has the tendency to show brush marks more easily. Both of the negatives can be avoided by a little planning and proper application. I have been burned by brush marks on my application a few times but these shortfalls are not permanent – if this happens then just wait for the polyurethane to dry and sand it off (while wearing your mask!).
What you will need
Gloves
Synthetic bristle brush
Avoid foam brushes – these will create more bubbles than a brush.
Step 1
Application Process
Wear your gloves and mask, try to vent the room as much as possible.
Yeah, I get it gloves are a nuisance, you have to remember to have them around and it seems so much easier to just dive into your project, after all – you have done this before and you will be careful, right???
The truth is that the amount of time that you will spend cleaning polyurethane off of your skin will greatly exceed the time you spend finding and putting on your gloves. Just do it, take the time and put them on.
Polyurethane, a petrochemical resin that contains isocyanates, is a known respiratory toxin, i.e., smart people think you should wear a mask, so do it. You should have air circulating as best as you can. My prior workshop was a basement and ventilation was at a premium. If my projects required a heavy dose of Polyurethane then I would wait until it was a nice day and take this step outside. For garage workshops just open up the garage and turn a small fan on to keep the air moving. Don’t point the fan directly at the project, this may lead to uneven drying and you risk blowing debris on the project that will be a nuisance to remove.
Step 2
Prepare your Surface
As a rule of thumb, start with rougher grit and get smoother. For general purpose, I sand with 80 Grit and then 120 Grit, always sand with the grain.
For some projects I will sand a third time with 220 grit. The higher the number the finer the grit. This results in a smoother surface. Be sure to clean your project with a damp lint-free rag. Remember, you are about to seal the wood – anything left behind will permanently be trapped on the surface. It’s important to take the time to make sure that all of the dust from sanding is removed before continuing to the next step.
New to Sanding?
Check out my guide to sanding here.
Step 3
Stirred, not shaken
One of the biggest enemies of the application process is bubbles, these pesky little devil drops can ruin a nice coat. One way to prevent them is to make sure that you do not shake the can. If you just made a last minute dash to home depot and slammed the breaks a few times then I would wait a bit before you dive in to the application process.
If you have bubbles at this stage you will undoubtably want to sand and re-apply – a time consuming mistake.
Step 4
Less is more
I hate this phrase but in this case less is more… effective.
One of my frequent mistakes was to over-apply the polyurethane. It is better to apply multiple thin coats. Heavy pours will result in dripping and potentially uneven drying. Both of which are eye sores that will need correcting. Polyurethane is tricky at this stage, oil-based products will go on milky and it will be tempting to brush things smooth – don’t.
Step 5
Do not overbrush
One of the worst things to do when applying polyurethane is to overbrush as it risks the finish drying with brush marks. Use your brush to apply a thin milky coat along the grain of the wood.
Use long straight strokes to cover the surface. Trust that things will dry smoothly. Avoid the temptation to try and “fix” area’s that look like they need more. Be careful along the edges, poly will drip and leave the sides of your project looking bad. This is more pronounced when incorrect (thick) layers are being applied. Polyurethane is very revealing, when looked at under light and from an angle (i.e., your phones flashlight) you should be able to see any pesky bubbles or drip marks. As mentioned above, do not overbrush but do your best to make sure that no bubbles or drips exist.
Step 6
WAIT TO DRY
Check the label for drying times – once the first coat has dried lightly sand the surface with 220-grit sandpaper (or finer). Do not push down hard, just a little sanding along the grain.
Step 7
Repeat
Clean the wood to remove sanding debris and repeat the application process two (2) to three (3) times.
It’s important to note that drying times and cure times are completely different. Your project will dry within a day but it will not fully cure for 20 – 30 days. This doesn’t mean that you can’t use the finished surface but it does mean that you should be cautious for the first month.
