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:
 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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