Urethane – Oil Based Paint vs. Water Based Finishes
The idea for this article on urethanes came from an individual I talked with at a home show in Victorville, California. I was thinking of doing a column on urethanes but this gentleman brought it to front and center. He was having trouble getting a urethane finish to last on his front door. I thought the column would be an easy one to research and write. Heck, how complicated can urethanes be? Well, I'll tell you what, I REALLY opened up a can of worms with this column!
The paint, excuse me, I mean "coatings", industry is very complicated. Paints and clear coatings are a witch's brew of many different ingredients and chemicals. They are blended in various amounts to create different properties. The terminology can get so technical so fast it can make your head swim. My advice to homeowners and professionals alike is to stay away from the chemistry aspect of coatings and concentrate entirely on the labeling and instructions about each product. If you carefully read a product label, you often will find clues as to how the product will perform. Isn't that convenient! Who would have thought it was that simple!
For example, just about every urethane label I have read has a paragraph heading "RECOMMENDED USE". It is vitally important that you read this paragraph before you buy the product! Don't buy a urethane designed for floors to put on your exterior door. There are big chemical differences in the products.
Urethane vs. Varnish?
Old fashioned varnishes are not much different than today's urethanes. The major difference is purely chemical. Urethanes contain an isocyanate (what the heck is that?). Evidently an isocyanate is a chemical chain that contains nitrogen, carbon, and oxygen. When combined with a polyol like traditional linseed oil, you get a urethane!
Urethanes outperform traditional varnishes because they resist chemicals, scuffs, and scratches. For the most part, urethanes are also slower to yellow or amber.
Sunscreens for Wood
Ultraviolet (UV) light has garnered lots of publicity in the past 5 - 10 years. We have finally figured out that it destroys just about everything -our skin, vinyl siding, certain plastics, colors, fabrics, wood, coatings, etc. The UV rays are invisible but evidently have the same impact as those very cool phaser pistols you see in "Star Wars" movies and such. UV radiation blasts things apart on a microscopic level.
Adding pigments or colors to urethane will help them to significantly resist UV degradation. What, you didn't know you can add pigments to urethanes? You bet you can! In reality, urethanes or varnishes are nothing more than clear paint. In fact, if you are trying to match stained woods, it can be done by blending pigments into a urethane. I have done it on several jobs to match new woodwork with older stained wood. The look is dramatic. After you achieve the perfect color, you then apply two coats of clear urethane over the pigmented coating. Without this protection pigments might wipe off when cleaning or brushed against with a white article of clothing.
Doors vs. Floors
Wood exposed to the elements and weather turns into a living, breathing "creature". In other words it moves. Urethanes formulated for these applications have to be softer or more flexible than a urethane applied to a gymnasium floor. A wood door or exterior sign expands and contracts. If the urethane can't absorb this shock, it will simply pop off the wood.
Maybe your exterior door urethane failed because you put a floor urethane on the door? It could have also failed because it had too little UV protectors. The UV light could have destroyed the surface wood cells. These cells succumbed to gravity and took the urethane with it!
Water & Oil Don't Mix, or do...
Great strides have been made in coatings technology with respect to urethanes. Water "based" finishes are now available. However, it gets confusing from here on. Why? Because there are two main groups of water based clear finishes. The same is true for oil based as well. The best performing water based urethanes tend to be those that "crosslink". However, guess what? It will be tough, if not impossible for you to find these in your regular paint store or home center big box store. Sorry.......
Crosslinking - What is That?
When you apply a regular water based paint finish, the water begins to dry. The pigment and resin (glue) particles are pulled closer together. Eventually these particles are touching one another very tightly as the last little bit of water evaporates. The particles stick to one another and the surface which was painted. Each particle maintains its own personality, however, as it sticks to the particle next to it. There is no chemical crosslinking in this scenario.
Old fashioned linseed oil, many oil based urethanes, and some water based urethanes, however, chemically react as they dry or "cure". What really happens is that the urethane actually turns into one GIANT molecule of urethane. The individual particles crosslink and become one. As you might imagine, this type of finish is much more resistant to moisture infiltration and chemical attack.
You will never see anything on a urethane label about crosslinking. It is too technical. However, you will see mentions on the label about "non-yellowing" properties. Many people don't like a urethane to amber or yellow. If you want this property, you are going to pay for it in performance. The urethanes that do not yellow are NOT crosslinkers.....that's not something to be proud about if you are a urethane.
Hardness of Floor Finishes
Urethanes used on floors need to be very hard. Hardness is directly related to the amount or degree of crosslinking in a urethane. The more crosslinking, the harder the finish. Other ingredients(the oils or resins used)also influence hardness as well. Suffice it to say that price will often be a clue as to performance. The urethanes that contain more crosslinking capabilities or chemicals almost always contain more higher priced ingredients. This means more cost to us, the end user. Check the labels to see if this isn't true! Look to see what type of performance claims the manufacturer makes on the label. Read closely and you might just be surprised.
Do you want a high performance urethane? If so, stick with an oil based one. They resist chemicals better, usually have a higher gloss, they do not raise wood grain, and are almost always more durable than water based urethanes. If you want the ease of rinsing your brush in water or want a non-yellowing film, you will not get long term performance. The choice is yours to make.