Why is My Poly Peeling?
Issue: Issue 279
Posted Date: 7/12/2011
have been using outdoor polyurethane on several projects. I am using
the Minwax brand. I have heard
from two people that have my projects that the urethane is peeling.
What do you think the cause of this could be? I wait no less than
12-24 hours before re-coating. I put 3 coats on as recommended and I
sand lightly between coats. I also use a rag dampened with denatured
alcohol to clean the sanding dust off between coats. I don't soak the
rag or the piece, but other than that, I follow the instructions on
the can. I don't think the cans of urethane that I use are very old.
I live in southern California so we don't get much rain and no snow.
I am perplexed at what I am doing wrong. Any advice you can give me
will be greatly appreciated. - Eric Levine
Marshall: I'm no
opponent of polyurethane; it gets plenty of use in my shop, too. But
maybe it's time to venture beyond poly for some of your future
projects. Both lacquer and shellac cure quickly and well in hot, dry
environments. They offer decent performance characteristics for
indoor projects, and they're certainly widely available at home
centers. Both also are easy to repair, and one layer bonds readily to
the next, unlike poly. Give these a try and you might find a whole
new avenue of viable coating options to add to your list.
Polyurethane finishes have tremendous durability characteristics.
Their major shortfall is their lack of good adhesion properties. They
don't stick well - to the surface they're on, or to the layers
themselves. Are you using an oil-based stain? If so, it might not be
"dry" before you're applying the finish coats. That can
influence inter-layer delamination (peeling). Another issue could be
relative to your location. Polyurethane requires moisture to set or
cure. Usually, 40 percent relative humidity is recommended. In
southern California, that might be hard to achieve. If you finished
your projects at a time when the humidity was very low, the finish
may simply not have been cured out well enough. What to do? Make sure
you have higher humidity in the air when you finish. Use a "swamp
cooler," add some mist, or do whatever it takes. Once the finish
has cured out, extra humidity would no longer be needed. Finally,
extreme heat/cool cycles can influence peeling. Is the project
sitting in or near a window or outside? That could be the fatal blow,