Mixing Oil Based Finishes with Water Based Finishes

 

Q. Can a water-based finish be applied over an oil-modified finish and vice-versa?

• A. Yes, a water-based finish can be applied over an oil-modified finish.  Likewise, an oil-modified finish can be applied over a water- based finish.  (Because both are urethane, they should be compatible.) The steps and concerns are about the same. First, the original product must be fully cured before you can successfully apply any water-based finish or oil-modified finish.  If you are applying water-based finish over a recent oil-based sand and finish job, you need to make sure that the oil-based finish has had enough time to cure.  Thirty days is typically enough time, but in very humid climates, 45 days may be necessary.  If applying oil-based over water-based, 14 days of cure time is necessary. If the original finish isn't fully cured, solvents will evaporate into the new finish and impede drying.  Several problems can result:  Alligatoring (wrinkling of the finish), bad adhesion, peeling, flaking off, splotchy areas and fish eye all can occur. Once you are sure that the original finish is fully cured, make sure you use manufacturers' recommended procedures for preparing the floor and applying the new finish.  Always use recommended cleaning products.  Because of possible contaminants on the floor resulting from things such as non-recommended cleaning products and maintenance procedures, most manufacturers do not guarantee a screen and recoat - whether you are using oil or water-based finishes.  On a related note, many contractors also ask if a water-based finish can be applied over a solvent-based stain.  The answer is yes.  In fact, the majority of stains used today are solvent-based stains, and often  waterborne finish is placed over them.  The key areas to pay attention to when applying a water-based finish over a solvent-based stain are: 

  • -relative  humidity 

  • -temperature  

  • -air flow 

  • -sanding procedures       

  • -application methods and tools

Always follow manufacturers' recommendations before applying their finish.  For best results when applying a water-based finish over a solvent-based stain, the room temperature must be between 65 and 75 degrees with a relative humidity of between 30 and 50 percent.  If the temperature and humidity are not within this range, the finish may have difficulty drying.  Dry time is critical.  The majority of finish problems are a result of the stain not being fully dry before application of the finish. Remember also that dry time varies with each manufacturer, so it is important to follow manufacturers' instructions. Low humidity and increased airflow will speed up dry time; conversely, higher humidity and decreased airflow will slow down dry time. In the final sanding, the grit used may also affect dry time. The finer the grit, i.e. 120 and 150, the more drying time may be slowed down.  This is due to the burnishing effect on the wood. However, if all excess stain is removed, this increased dry time should be minimal. A darker stain or a white stain may take longer to dry.  Medium- colored stains dry in less time. It takes from four to 72 hours for stains to dry. (Again, check with manufacturers' recommendations.) There is not a scientific way to determine if the stain is dry, but most contractors use a dry, clean, white cloth to rub the stain, detecting any stain transfer onto the cloth.  Also, some manufacturers recommend buffing with a white pad before applying finish.  When you start buffing, check the pad bottom frequently for transfer - if it is stained, your floor isn't dry yet. Follow manufacturers' instructions as to coverage rate and uniformity of flow.  Application tools normally used are a T-bar with a synthetic applicator, synthetic roller, pad-type synthetic or lambs-wool applicator, and hand brushes made specifically for water-based products.  These hand brushes can be made of nylon or of a similar synthetic material. The manufacturer instructions must be followed as to the number of coats for residential and commercial applications. Generally, three to four coats are sufficient for a typical residential application, while commercial applications may require four to five coats, including a sealer.

 

Technical questions answered by former NWFA technical director Daniel Boone(1994-2000)

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