Spraying Lacquer in Challenging Environments
Here's some helpful tips from the lead chemist of Mohawk Wood Finishing products on how to get outstanding finishes in less-than-optimal conditions.
Viscosities should be taken at a specific temperature for consistency. If always tested and recorded at 77° F. for instance (There’s no magic behind this temperature, it’s simply a value chosen to use for consistency and it’s easier to warm material a few degrees than to cool it). If the coating is cold the viscosity will be higher and atomization is affected which may lead you to thin or reduce the coating. While this may help, you’ll be spraying at a lower solids content which decreases dry film thickness when applied at the same wet film thickness. This is why your finishes may appear hungry in the winter months as compared to those in the summer. By simply warming your material before you spray you can restore the viscosity without reducing and maintain the solids content. By planning ahead and staging your coatings in a heated area they’ll be ready for use with less or no modification.
As temperature rise the viscosity will drop and atomization may improve. Many coatings flow better when warm and some are designed for hot spray. Unless specifically directed to heat your coatings you should not do so or you may experience more runs or sags and dry times that are too fast.
Evaporative finishes are formulated to dry at a controlled rate. The faster solvents will come out of the coating rapidly aiding in a faster set or parking of the wet coat. The middle range solvents remain in the finish longer helping to maintain a wet film to allow subsequent passes to wet back into the coat preventing lap marks by allowing overspray time to fall into a wet film allowing it to melt back into the coat. As the middle range solvents leave the film the slower tail solvents continue to help with intercoat adhesion and prevent blushing in lacquers as they provide an for moisture before the film dries and traps a blush. This over simplifies the solvent blend and its purpose but the point is to show how the drying process is meant to be gradual and controlled to accomplish a proper film coating.
Coating products are designed for optimal performance within the manufacturer’s desired parameters but various restrictions force changes in formulations that challenge usability. As VOC levels change many solvents that help with a controlled evaporation rate are reduced or removed completely to achieve lower VOC levels; they must be replace by other solvents which may not be as helpful to the flow process. You may also find that adding retarders or other modifications is not possible while maintaining the VOC limit directed by your area or circumstances. Be aware that few solvents can be added without raising VOC levels. Consult your vendor for recommendations.
As discussed earlier, cold temperatures will cause a rise in viscosity and heat will lower it both of which affect atomization. As temperatures rise the evaporation process is accelerated which can cause a coating to dry as you’re spraying which allows overspray to fall on drying finish without a chance to melt back onto the coat. To compensate, finisher’s tend to spray heavier coats in an attempt to maintain a wet film which results in higher film build that exceeds the recommended maximums. Because flow is affected and solvents are evaporating so quickly this heavy spray doesn’t tend to run which further encourages heavier applications that can lead to cracking issues when maximum film thickness is exceeded. Adhesion becomes a concern when the coatings are drying faster than they can flow and wet into the wood fibers, sanding scratches or melt back into the previous coat. Testing for adhesion is always advised particularly when your finish is drying prematurely.
Humidity, Temperature & Dry Times
Stock finishing products are formulated to fit a wide range of conditions but most can be modified when used outside of their designed range as VOC regulations allow. Slow evaporating retarders can be added in periods of high humidity when blushing is likely. Middle range solvents help maintain a wet edge when they’re drying too fast particularly in hot conditions. Slower tail solvents or blush retarders, which can remain in the film long after the finish is safe to handle, will delay flipping or stacking door or panels so using them in excess will slow production and can lead to blocking or marking. By combining a middle range retarder with an appropriate amount of blush retarder you will address the specific points in the evaporation process that are causing problems. With proper adjustments you can maintain flow and give your coating a chance to make a bond to ensure good adhesion. This is a big concern when coatings are drying too fast; adhesion tests should be performed when conditions are affecting dry times and adjustments should be made to correct the issue. Cold damp conditions will slow dry times and the use of retarders should be reduced or eliminated to prevent delays. If blushing is occurring you should use the minimal amount of blush retarder required to allow it to dissipate. Reducing viscosity with faster solvents can help speed drying times when cold temperatures are retarding your dry times. VOC restrictions will dictate which modifications you can make to your product as well as the compatibility of your finish system. You must make sure that you do not exceed VOC limits when modifying products and use the appropriate additives for both compatibility and compliance.
Waterborne Coatings & Temperature
Many waterborne coatings cure by a coalescing process where resins form a film once the water has evaporated leaving them in concentration with the coalescing solvent. While in its wet state with the water the process is delayed, as the water evaporates the film is formed and then coalescing solvent evaporates as well. In temperatures below 60° F the water evaporation slows and the coalescing solvent can evaporate faster than the water and the coating won’t cure properly. In hot conditions the water and solvent are driven out of the coating too fast causing dry spray and adhesion concerns as well. Waterborne coatings have a tighter temperature range and there are fewer modifiers as well. Anytime conditions threaten the finishes performance and modifications can’t be made you should improve conditions or delay if they are out of your control.
It’s important to understand how conditions affect coating performance and to make corrections to prevent failures that can be very costly. Regardless of conditions you still have to stay within specifications of your finish, regulatory compliance, ensure proper adhesion and achieve an appealing film finish. It is the finisher’s responsibility to test their processes in all conditions and make adjustments as needed to maintain a quality product.