When applying exterior wood finishes, finding the perfect weather conditions can be a challenge. 72 degrees Fahrenheit with 50% relative humidity on a partly cloudy day with no rain in the forecast... we might get one or two of those in a Minnesota summer and chances are some other event will already be on the calendar.
Finding label-ideal conditions may be a challenge, but temperatures and other weather conditions are important considerations for best coating performance. Besides avoiding precipitation, temperature is one of the most critical considerations. Beyond only checking the air temperature, almost more important is the surface temperature; after all, that’s where the stain will be applied and it needs ideal conditions to get the best performance. Air temperature only tells part of the story, but in the shade or in the sun the wood surface can vary greatly.
On one project staining rough-sawn cedar in early May, the air temperature at 9am was 48F and climbing. On the northeast corner of the house, where one side was in the shade and the other was in the morning sun, there were immediate differences. The north shaded side matched the air temperature of 48F and the east has already climbed to 70F. An hour later at 10am, air temperature has risen 4 degrees, but the sunny surface has risen 20. Most exterior finishes are ideally applied around 50-90F, so there was a situation where the air temperature and shaded side were at the minimum of 50F, but the surface on the sunny side was already at its maximum of 90F. The consideration of temperatures in the sun can be even more pronounced in the heat of peak summer. Later that afternoon, the air temperature was in the low 60’s, but the south sunny side was over 100F and unsuitable for staining. In that situation, staining on the south side was ceased until cloud cover returned and the surface had cooled to avoid flash-drying of the stain.
The air thermometer may look ideal at 72F, but when measuring the surface of a sunny side, it may read well over 100F. Applying stains and sealers on surfaces that are too hot or too cold will result in poor penetration or adhesion, and ultimately the coating may fail. Handheld surface thermometers are as easy to use as a laser pointer. Approximately $30-$100 to purchase, they are undeniably cheaper and easier than stripping off failed stain and reapplying it all over again.
Without a surface thermometer, the layman technique would be to only stain in the shade or under cloud cover, with air temperatures at 50-90F (or whatever range is given on the product label).
Questions on maintaining the legacy of your wood structure? Give us a call- 1-800-359-6614
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