When installing wood floors, builders, installers and DIYers need to consider that wood finishes also contain moisture. In this case, “moisture content” (MC) takes the forms of oil, water, or urethane. Installing wood floors with healthy and durable finishes means understanding the moisture interactions between both the floor and the finish.
Types of Wood Finishes
Wood floors that were installed prior to the mid ’60s were likely finished with varnish or shellac. It’s fairly simple to check. Find an inconspicuous place in the floor, and scratch the surface with a coin or other sharp object. If the finish flakes, it is probably shellac or varnish: floor finishes of a bygone era. Natural varnish (made with vegetable oils) made way for vinyl-alkyd, and that made way for urethane.
Following are the different types of wood floor finishes currently available:
- Oil-modified urethane is generally the most common surface finish and is easy to apply. It is a petroleum base with a blend of synthetic resins, plasticizers and other film-forming ingredients that produces a durable surface that is moisture-resistant. It is a solvent-based polyurethane that dries in about eight hours. This type of finish ambers with age and is available in different sheen levels.
- Moisture-cured urethane is a solvent-based polyurethane that is more durable and more moisture-resistant than other surface finishes. Moisture-cured urethane comes in non-yellowing and in ambering types and is generally available in satin or gloss. These finishes are extremely difficult to apply, have a strong odor and are best left to the professional.Curing of this type of finish is done by absorbing minute quantities of moisture vapor from the air, which causes them to harden. The curing process is very dependent on relative humidity.
- Water-based urethane is a waterborne urethane with a blend of synthetic resins, plasticizers and other film forming ingredients that produces a durable surface that is moisture-resistant. These finishes are clear and non-yellowing and are different sheen levels. They have a milder odor than oil-modified finishes have, and they dry in about two to three hours. Water-based urethanes are generally more expensive.
- Conversion-Varnish Sealers-(Swedish Finishes) Are two-component acid-curing, alcohol-based sealers. Because of their origin (country), conversion varnish sealers are often referred to as Swedish finishes.
- Penetrating Sealers are spread on the floor and allowed to penetrate, and are solvent based. The excess sealer is removed with rags or buffed in with synthetic or steel wool pads. This type of finish often has a color and can be used to stain and seal the wood floor. Penetrating Oil Sealers are made from tung or linseed oil, with additives to improve drying and hardness.
- Paste Wax is the oldest, and in some ways the best. Wax is the easiest to apply, least expensive, fastest drying, and easiest to repair, and with proper care it will survive forever. Wax over a penetrating stain, and the system is in the wood so you wear the wood, not the finish. Wax is spread in thin coats for surface protection after the stain and/or sealer is applied, then buffed to the desired sheen.
- Varnish – Vinyl-alkyd varnishes have superseded natural varnish made from vegetable oils. This product was commonly used before urethane finishes were introduced.
- Lacquer – The flammability and incompatibility of this floor finish is NOT recommended by many manufacturers. This finish should avoided.
- Shellac – This product (natural shellac) contains wax and is not widely used for top coating in today’s wood flooring market. Dewaxed shellac is being used more and more for a wood floor sealer.
What to Remember
Any wood flooring finishing choice involves moisture, because liquid is the form of application when installing wood flooring finishes (with the exception of the wax finish). In addition, all wood has a certain level of MC, water vapor within every single wood cell. Installers must remember that water, oil, synthetics and stains all interact with each other when installing wood floors with any finish.
The relative humidity (RH) and temperature of the environment also influences the durability of wood flooring. When RH is low, wood loses MC as it evaporates; when RH is high, wood gains MC from the surrounding air. Wood flooring installers aim to achieve the wood flooring reaching its equilibrium moisture content (EMC) previous to installation by leaving the flooring at the installation site to adjust to ambient conditions. EMC is the balance between the wood’s internal MC levels with that of the surrounding environment. Wood flooring is more likely to fail (and thus, wood finishing with it) when installers or contractors fail to ensure that the wood flooring has reached its EMC before installation and finishing.
How to Succeed
Recognize the continual need to measure and balance the MC of your wood flooring and you enhance your chance of successfully installing and finishing wood floors. Ensure proper equilibration time for new flooring and use an accurate wood moisture meter to prevent potential MC-related problems.
Drying time is the key. Successful wood flooring installers often keep room temperatures between 60 and 80 degrees with RH between 30 and 50 percent. Outside these parameters, wood finishes may not dry properly: the definition of trouble when finishing wood floors. Remember: wood stain must dry completely before applying wood finishes, because the majority of finish failures begin at this stage. Check the manufacturer’s RH specifications before applying wood finishes. Finally, low RH and increased airflow help wood floor finishes dry even though those conditions may not be ideal for the floor’s longevity. But don’t create a long-term disparity between the ambient conditions and the installation conditions of your environment or your floor will suffer for it.
A wood moisture meter is an excellent way to measure and confirm the wood flooring’s MC prior to finishing. For waterborne wood finishes, consider Wagner Meters L607 (discontinued) wood moisture meter. The L607 reads the MC from 5 to 30 percent on finely-sanded surfaces to let you know when your finish is ready for a second application. No need to let excessive moisture in the wood flooring ruin a great wood finish.