LACQUER, Finishing Secrets No. 11
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NOTE:
This discussion does not include the water-borne lacquers. Until recently, these products resembled real lacquers in name only. They didn't have the same clarity and brilliance, they didn't enhance the grain very well, and they reflected a dead bluish color that wasn't at all appealing. In use they didn't burn into the previous coat, and often had adhesion problems. That is changing with some new products that are entering the market. I have seen them used on turned wood, and the results were good. I will discuss them after I have some experience with using them.
Introduction
A spectacular grain “picture” calls for a spectacular finish, and a polished lacquer is the ideal choice for such a piece of wood. Lacquer has the unique ability among finishes to build a durable crystal clear film that gives depth and clarity to the wood surface, while accenting the grain and any chatoyance that is present, and having a sparkle and brilliance that no other finish can match.
Lacquer is the easiest of all finishes to apply, but it is also the most difficult to do well because it is the least forgiving of anything that we can put on a piece of wood. Lacquer will add some new words to your finishing vocabulary, “blush”, “fish-eye”, “blister”, “sag”, and “run”; and you will learn to invoke the wrath of God in several languages. Don’t let this scare you away because they are problems that we can easily work around once we know what causes them.
Lacquer can be applied with either spray or brush. The results from brushing can be equal to spraying, but it will take a lot more time and hand-work to get there. I prefer to use air because it is faster. Brushing is discussed after we have explored the spray application.
This discussion is a “what works for me” approach to using lacquers. It should help the novice finisher get past the trial-and-error part of learning a new finishing technique. The missing ingredient is the experience with using lacquer and developing the motion and rhythm for using the spray gun or brush. That takes the practice, and that I can’t give you.
Equipment For Spraying
Don’t write-off a sprayed lacquer finish because you have been led to believe that large and expensive equipment is required. Nothing can be farther from the truth. A good brush costs more than a good spray gun. A “best buy” is the siphon-type detail gun that is available from Harbor Freight retail stores for something like $15.00. The same gun will cost almost twice that from Home Depot or Lowe’s.
A compressor rated at 4-CFM at 100-PSI can support spraying our turned wood. That is not a large compressor.
I do use an in-line air filter, but use neither a water-separator nor an air-dryer. My approach to spraying lacquer is that if water condensation is a problem, the atmospheric conditions are wrong, and I shouldn’t be doing it anyway.
Moisture and “Blushing”
“Blush” is a white cloudy appearance of the finish that is caused by water vapor that condenses on or under the finish as is applied or dries. In either case the water is absorbed into the finish and forms the characteristic cloudy appearance.
Understanding the problem can keep us away from the situations that will affect our finish.
All air contains water vapor. The amount of moisture is defined by the “Relative Humidity” (RH), and the warmer the air, the more moisture it will hold. The other important temperature is the "Dew Point" and that is the temperature where the air becomes saturated and the moisture will start to condense. The secret to spraying lacquer is to prevent either the finish film or the air that carries it to the surface from cooling to below the dew point.
Rapid evaporation of the thinners in the lacquer cools the surface film. If the surface temperature cools to below the dew-point temperature, the water vapor in the surrounding ambient air will condense on the surface as it dries.
To some degree, “blushing” can be controlled through the use of “retarding thinners” that evaporate more slowly than the normal lacquer thinners. This prevents the evaporative cooling component.
Water vapor can be a problem in the air stream that passes through the compressor and spray equipment. Air gets hot as it is compressed, and cools as it expands through. If there were no intermediate cooling between the compression and the expansion, it would return close to its ambient temperature. If the hot compressed air is allowed to cool, such as in an air-tank, it will then expand to a temperature that is lower than what was ambient before it was compressed. When this new lower temperature is below the dew point of the air, the water vapor will condense.
In the worst condition, the condensation will form water droplets in the air stream, making craters called “cat eyes” in the surface film. Even when the air does not cool sufficiently to form water droplets, it can cool the lacquer that it is carrying to the surface, adding to the cooling of the film that takes place from the evaporating thinners.
Water droplets that condense in the air stream can be removed with mechanical separators that are installed in the air lines, but the cooling effect on the finish product that it carries remains.
A turbine powered HVLP (high volume – low pressure) spraying equipment eliminates both the water and cooling of the air stream because the air pressures are low, and the air exiting from the gun is close to ambient. A conversion gun does not because the air went through a high pressure compressor and tank before expanding through the conversion gun. This compression and expansion cycle lowers the air temperature and makes it vulnerable to water problems. The cheaper solution is to obey a few rules that keep us away from the conditions that will cause problems.
Atmospheric Conditions
Lacquer is not as forgiving of our working environment as other wood finishes. Because of inadequate ventilation in our shop/studio, most of us have to spray lacquer outside, or in an open doorway, where we have no control over the surrounding air. We can all but eliminate many of our problems if we obey two simple rules.
The amount of water vapor in the air can be measured, as well as the probability for it condensing into droplets or “blush”. Knowing the ambient air temperature, its “relative humidity” and the “Dew Point Temperature” we can determine whether any day will be a good one for spraying lacquer. Having a wet/dry bulb thermometer or an electronic RH gauge is always the most accurate, but current Weather Bureau information is usually close enough. This information is available from the Weather Channel website or other sources for similar information.
While there is no single set of values that will insure successful spraying, I have learned that the following guidelines can be useful.
The “Rule of 65”
I have learned the hard way that lacquer should not be sprayed when the temperature is below 65-degrees, or the Relative Humidity (RH) above 65%.
Violation of this rule can result in “blushing” (cloudy appearance) of the surface film from trapped water vapor. If we deviate too far, water droplets will be blown on the surface. The range of temperature and RH for brushing lacquer is broader because of its slower drying rate and the fact that we usually try to do it inside the shop.
The “20-degree Rule”
It is safe (usually) to spray lacquer when the dew point temperature is 20-degrees or more lower than the ambient temperature.
This is an additional guide when it might seem impossible to spray lacquer if we were to obey the “Rule of 65”. In other words, I might get away with spraying lacquer or other finishes with fast drying thinners at an ambient temperature of 100-degrees, if the Dew Point was below 80-degrees, regardless of the RH.
I would improve my chances of successfully spraying the lacquer by using a small capacity tank that would allow very little cooling between compression and expansion of the air. This may permit us to work with a DP that is closer to the ambient temperature than the recommended 20-degrees F.
An example of when not to spray lacquer would be this morning, 10:00AM in mid-October, as I am writing this in North Idaho. The Weather Channel says the temperature is 43-degrees, the Dew Point is 37-degrees, and the Relative Humidity is 81%. I don't need any more information thyan that to tell me that I will have a problem with condensing water everywhere. I am nowhere close to the "65 Rule", and the Dew Point is a lot less than 20-degrees below the ambient temperature.
Later this same day things look better. The temperature is 60-degrees, 43% RH, and a 36-degree Dew Point. While the temperature is below 65-degrees, everything else looks good.
Other Conditions
There are several other conditions that can cause problems with our sprayed lacquer finish if we ignore them.
Bugs and dust
Bugs love lacquer. Don’t try spraying a finish when there are a lot of them about. And, try to choose a calm day to reduce problems with airborne dust. Contamination by bugs and dust particles is more noticeable in a lacquer than any other finish that we can put on a piece of wood.
Sunlight
Do not expose the work to direct sunlight while the lacquer is still wet. To do so will usually result in “blistering” and bubbles in the surface film as it dries.
Oils and Silicones
Do not contaminate the wood or film surface with oils of any type. These oils will cause a small circular dimple in the surface film, called a “fisheye”. To insure a clean surface, do not use sandpaper that has anything added to its surface for easier sanding.
Do not use any wax containing silicones anywhere in the shop.
Keep your hands washed and dry, and avoid getting any perspiration on the wood surface.
Steel wool must be “oil-free”. I recommend either the Libron brand that is available from Craft Supplies or purchasing steel-wool from an automotive paint supplier. Don’t trust the package label on commercial steel-wool because “oil free” is a relative term. If in doubt, rinse it two times in clean Mineral Spirits and thoroughly dry before using. Then test it by spraying lacquer onto a wood surface that has been vigorously rubbed with the cleaned wool.
Shop Dust
When we spray or brush lacquer in the shop, we can control the temperature, RH, sunlight and bugs. But, shop dust is a larger problem. A clean shop is the obvious solution. Spraying the lacquer when first entering the shop, before the dust has become airborne, will help.
Ventilation
There are three (3) new problems to solve when spraying lacquer in the shop – ventilation, ventilation, and ventilation. Spraying indoors is not recommended unless you have a dedicated spray booth, a good supply of fresh air, and a full-face respirator with carbon filter cartridges. A spray-booth is recommended.
Ventilation can suck all of the warm air out of the shop in the winter.
Go to the next page for a discussion of the different lacquers.
This page was last updated: December 10, 2011
