What You Need to Know About Polyurethane

What is Polyurethane?

Before I discuss what you need to know about polyurethane I want to start by saying that I’m only referring only to oil based products in this article.  I will cover water borne finishes in a separate article in this wood finishing class.

Polyurethane is a synthetic polymer.  In layman’s terms, it is a type of plastic that can be formulated in many different ways. You can buy foam seat cushions that are made from polyurethane, you can buy “plastic” gears, or wheels that are made from polyurethane, and of course, it can be formulated into a resin that is used in a very popular wood finish. Let’s talk more about that wood finish!

Polyurethane is a film forming finish. This means that after the first coat soaks in and cures to seal the surface that subsequent coats will build on each other to form a film over the surface of the wood.

Polyurethane is actually an alkyd varnish with polyurethane resin added to it. It also contains many other ingredients such as drying agents, UV inhibitors, etc. But, it has two primary components:

1. Resin(s), or “binders”. In this case these are mostly some ratio of alkyd and polyurethane resins. 
2. A carrier to help the resin flow and penetrate the wood. This is typically a solvent like mineral spirits, naptha, or something similar.  Often, the manufacturer will add different types of solvents to a particular product.

Polyurethane is a “reactive” finish.  Meaning that it doesn’t cure by evaporation, or “drying”, but that it cures by reacting with oxygen.  It is true that the solvents must evaporate, but it only cures to a hard surface by reacting with oxygen.

There are a few reasons why this is important. It means that a polyurethane finish is more difficult to repair because once fully cured it so tough that it is impervious to the original solvents.  It also means that multiple coats of polyurethane lay on top of each other in layers rather than melt into one solid film like shellac does.  Consequently, if you sand through multiple layers of a polyurethane finish then you will likely get visible lines where the sanded area transitions from one layer to the next. 

Pros and Cons of Oil Based Polyurethane

Polyurethane is a great wood finish. There are other great options, but polyurethane is far and away the most commonly used finish among woodworking enthusiasts. And, there are good reasons why it is so popular:

• Polyurethane is inexpensive (Usually)
• Like all oil based finishes, polyurethane tends to darken and add richness to the color of the wood.
• Polyurethane is easy to find
• Polyurethane is one of the most durable finishes that is available to the general public
• Water borne Polyurethane has excellent durability, but oil base  Polyurethane is generally just a bit more durable that its water borne cousin.
• Polyurethane is available from a matt sheen up to a very glossy sheen and everything in between.
• Polyurethane can be brushed on, wiped on with a rag, sprayed from an aerosol can, or sprayed from dedicated spray equipment
• Polyurethane is available in different colors
• Polyurethane is easy to apply, and it’s even easier after you learn the best ways to apply it

You can see that there’s a lot to like about polyurethane! That said, oil based polyurethane has a few drawbacks:

• Oil based Polyurethane has a strong odor.
• Oil based Polyurethane is flammable
• It is more difficult to clean brushes that are used with Oil based Polyurethane. 
• Some people claim that it has a “plastic” look.  There is some truth to this, but that can certainly be minimized.
• Oil based Polyurethane is very high in VOC’s
• Applying most wood finishes can be hazardous to your health, and you definitely need to protect yourself when using oil based Polyurethane 
• Oil based Polyurethane dries and cures slower than some other finishes.

These don’t mean that polyurethane is a bad finish.  It just means that these are a few characteristics that woodworkers should be aware of before using it. Because no single wood finish is perfect in all respects. For example, the absolute most durable ones are hard to repair, but the most repairable ones are not the most durable.  

What is the “Best” Polyurethane to Use?

There are lots of different polyurethanes on the market. Every polyurethane product has a slightly different formulation.  These differences mostly affect how well it flows out or how quickly it will cure.  These differences can also affect chemical resistance and durability. In actual practice the differences between different brands of oil based polyurethane are slight.

I’ve seen tests that show General Finishes Arm-R-Seal (polyurethane) is a bit more chemical resistant than the Minwax equivalent. But, the differences are small and they only show up in extreme chemical and durability tests.  In normal day to day use you are unlikely to see a difference.  I like General Finishes products, but Arm-R-Seal is more than double the cost of Minwax and I can’t justify spending that much more for it.

I know I just said that most polyurethanes are all about the same.  And, I believe it’s true, but I do have a personal favorite. I prefer Minwax Wipe On Polyurethane for those times when I want to use an oil based polyurethane.

I  like Wipe On  Polyurethane because it is so easy to apply. 

Many people say that adding a solvent like mineral spirits to regular polyurethane makes it the same as the pre-mixed wipe on polyurethane.  This may be true, and I will often add a solvent to regular polyurethane to thin it down and make it easy to apply.  That said, my first choice is the pre-mixed wipe on polyurethane.  It is similar to thinning down the regular version, but the pre-mixed seems to work just a little bit better.

Tip: When I’ve thinned polyurethane with regular “smelly” mineral spirits it seems to flow just a bit better than when I use “odorless” mineral spirits.  If you live in a state that doesn’t allow the sale of “regular” mineral spirts then use whatever solvent is available.

Prepping the surface:

As with any wood finish, you need to prepare the surface of the wood before you apply the finish. This means to first sand or plane the surface smooth, and to ensure that there are no dents or scratches, and that the surface if free of oil, dirt, and debris. 

With a film forming finish like polyurethane it’s usually not necessary to sand the surface with anything finer than 180-220 grit paper.  This is because as you build the finish with multiple coats that the finish will fill in the sanding scratches to the point that they are not be visible.

But, there is an exception to the 220 grit guideline. Sometimes you want to use poly for its excellent chemical resistance, but you want it to look more like an oil finish than a film finish. To get that “in the wood” look you might apply just a few coats of thin poly to your project. In that case the final finish would be too thin to fill in the sanding scratches, and you would want to sand the wood with finer paper, maybe up to about 400 grit.

Applying Polyurethane

You can apply polyurethane in several different ways.  You can brush it on, you can use a spray can, you can use rags, or you can use dedicated spray equipment.

My favorite way to apply polyurethane is with rags.  I think rags work better and they are easier than using brushes.  They are inexpensive, they get great results, and you don’t have to clean them!  I like using rags so much that I wrote an whole article on how to use rags to get a perfect finish.

Of course, brushes are also a good way to apply polyurethane.  Natural hair brushes are recommended for oil based finishes. One caution about brushes is that it is tempting and easy to apply it too thick. 

Spray cans are an excellent way to apply polyurethane to small projects.

Dedicated spray equipment is probably the best way to apply polyurethane, especially to larger projects.  But, it’s not easy to use spray equipment in a small shop.  You need good ventilation and a space where you don’t have to worry about overspray getting all over everything.  Spray equipment is more of an investment. Not only does it require a financial investment, but it also requires an investment of time to gain proficiency. That said, you can get the best finishes in the least amount of time with spray equipment.

Sanding Between Coats of Polyurethane Isn’t Always Necessary

Multiple coats of polyurethane will generally adhere to each other with or without sanding between coats. This realization was actually quite a surprise to me.  I had always heard that sanding scratches were needed for polyurethane to “mechanically” bond to itself, but this is a myth.  I have applied many layers of polyurethane without sanding between coats and adhesion between coats has never been a problem.

This myth is so engrained in the woodworking community that it is considered an absolute truth, like water is wet and snow is cold, but it is not true.  I even conducted adhesion tests with different types of polyurethane to verify that it will adhere just fine without sanding.

There Are Times When You Should Sand Between Coats of Polyurethane

If you want a nice smooth finish then you should only apply polyurethane to a smooth, clean surface.  This is true when applying it to raw wood and it’s also true when applying a second or third coat.  If the previous coat has dust nibs, runs, or any other sort of roughness then you should sand that out before applying the next coat.  It is often recommend to sand between coats with 180 or 220 grit paper, but it’s easy to remove too much of the previous finish with sandpaper that is that coarse. It’s hard to build a finish if you sand most of it off between coats! I will sand lightly with 400-600 grit sandpaper between coats to remove dust nibs and other minor imperfections.  If the previous coat is already smooth then I won’t sand at all.

A final thought

I hope that you are enjoying this Free Wood Finishing Class. I put a lot of time and effort into this, but I don’t mind as long as people are enjoying it. I have so much wood finishing content that it will take me a while to post all of it here on my website.  Please let me know what you would like to see next!