It seems that one of the topics I get a tremendous amount of emails about is when folks are having issues due to the heat and humidity. A huge number of woodworkers spray their finishes outside and in doing so, are at the mercy of uncontrolled temperatures. Spraying in these conditions can be risky business.
Most finishes simply do not like to be force dried. When spraying in direct
sunlight the surface of the finish will skim over leaving the underlying finish still soft. The finish can then blister; this is most predominant in solvent base finishes. The surface dries and the air that is trapped in the pores of the wood cannot readily escape.
As the air rises it must now break through the dried film and forms a blister. It is a situation where the direct sunlight is the culprit and being able to shade the surface makes a huge difference.
Another issue as a result of the surface drying rapidly is called “blushing”, particularly in hot humid areas. Again, the moisture is trapped within the finish as with blistering. The solution once again is to shade the surface from direct sunlight. If using compressed air, make sure your air is dry and filtered. Turbine systems are the best solution to ensuring clean dryer air.
Lately, the biggest issue is being able to wipe on any non-oil dye or stain in hot/humid weather. Oil-based stains, because they dry so slowly, are usually not an issue. Gel stains, because of their heavier viscosity, dry quite rapidly. Water base stains and dyes can be very problematic. Alcohol/Lacquer dyes and stains can be all but impossible.
Here are a few tips to help when you find your dye/stain drying too rapidly and not allowing you to wipe evenly.
For oil base gel stains, have a cloth damp with mineral spirits ready. Work in as small an area as possible. The objective here is to be able to wipe the stain on and off before it sets. The mineral spirits will help to prevent the stain from drying as fast, but it can also produce a lighter color so often two coats are required. Just be sure to let the first coat dry thoroughly or you run the risk of the second coat softening and pulling off the first coat.
Waterbase dyes and stains can be controlled to some degree the same as the gel stain. Use a dampened applicator which can help, as well as working in as small a section as possible.
In the case of dyes, premixed dyes are not as good as powdered dyes that you mix yourself. Pre-mixed dyes have chemicals that speed the drying process and they are not typically just mixed with water. Powdered dyes that you mix yourself, because it's simply water, seem to dry slower giving more work time.
We have experimented with utilizing "Floetrol" which is a paint additive used for slowing the drying process in order to improve leveling of water base paints in water base dyes and stains. Floetrol is available at most hardware and box stores where latex paint is sold. We have found it to be very helpful. Our normal mix is approximately 1 ounce to 1 quart of dye or stain. We have used as much as 2 ounces without issue. We have tested this under numerous water base finishes with no issue. But as always, it is a good idea to test any formula on a piece of scrap to ensure total compatibility.
Without question one of the best ways to apply a dye or stain in hot weather is to spray it liberally, again in as small sections as possible, then immediately wipe it back. The spraying allows for a rapid application of the colorant, thus giving more wiping time before it starts to set.
If spraying is not afforded then you want a good stain pad; it will hold more liquid and allow you to cover more area faster. Trying to take a brush or foam brush and apply a colorant in hot weather is all but impossible. The brushes simply do not hold enough material to allow you to evenly wet any sizable area. Just the short time of having to reload the brush is allowing the dye or stain to start to set.
You will also find that using paper towels to wipe off the excess is very
beneficial. They absorb rapidly and allow you to wipe the surface quicker.
To cut to the chase, the bottom line is you have to move quickly before any of the colorants begin to set. It's not a bad idea to have someone helping you wipe off almost as fast as you wipe on. The general rule of thumb of allowing the dye or stain to set for a few minutes simply put doesn't work in hot weather.
As stated above, my preferred method is to spray it really wet and start wiping as fast as possible, and in all cases avoid direct sunlight.