The relatively new generation of hardwood floor finishes and the most commonly used nowadays, polyurethanes (sometimes called urethanes) are used only as a protective layer over stained or unstained floors. Their excellent durability and great looks makes them the sealers of choice in todays’ hardwood floor finishing and wood floor restoration projects.
There are four major types of polyurethane finishes: water-based, oil-based, moisture-cure and acid-cure. Because of their different chemical structure, they behave differently, so you need to know what type your finish is before you apply, reapply or remove it. Choosing between these finishes depends on the specifics of your hardwood floor and your personal priorities. Here are a few general properties of polyurethanes.
If you prefer a floor with lots of shine and pizazz, go with oil-based polyurethane, the long-standing hero and the cheapest option in the world of urethanes. Oil-based polyurethane is easy to apply even by a DIY-er because of its self-leveling properties. However, its VOC emissions are high, and the finish may yellow over time (it develops an amber tint). Keep your horses in check, too: oil-based urethane is slow to dry, so applying more coats may take you more than one day. It requires up to 12 hours before foot traffic is possible versus the 2-3 hours necessary for the water-based formula. This means that it takes more than one day to apply the 2-3 coats required for total coverage. The smell may drive you out of the house and may take a few weeks to completely disappear.
Its water-based formula makes this type of finish more eco-friendly than other urethanes. It is low in VOC fumes and has a milder smell. Water-based polyurethane is easy to work with and dries fast, so a one-day application is possible. However, it is thinner than other urethanes, so it may need more coats. This formula is not flammable, which is an excellent quality when it comes to sanding and application. As for the look of your floors, water-based urethane dries clear and remains clear, so you’ll have no surprises down the road. What you don’t get from it is the rich look of the oil-based urethane – it is sometimes so clear that it’s even difficult to notice it’s there. If you want a subtler look, go for it.
When extreme durability is a priority, moisture-cure polyurethane is an excellent option, generally applied over alcohol-based wood floor stains. This formula uses the moisture in the air to cure, dries fast and does not yellow. Because of its strong fumes and odor, it is less popular today and mainly used for high-traffic, industrial or commercial projects where resistance to abuse is essential. Moisture-cure polyurethane cannot be applied over other types of wood floor finishes, so sanding is absolutely necessary in case you refinish your floors. Drying time depends on relative humidity, very dry or very humid climates may cause problems. This makes application difficult and not appropriate for the DIY-er. Applying this type of finish must be done by wood floor specialists equipped with full-face respirators and full body suits.
Going by names such as Swedish finish or conversion varnish, acid-cure polyurethane is extremely durable, resistant to accidental encounters with chemicals and does not yellow. Great for showcasing the beauty and grain of your hardwood floors, its fast-drying formula makes for a quick and efficient application.
But there are trade-offs. The very strong odor and the high VOC level require professional application by wood floor specialists, protection-gear with full body suits, good ventilation and leaving the premises, pets included. This is a flammable formula, so extra care must be taken.