Hot humid weather can play havoc in your finishing room. High temperatures can lead to an increase in orange peel, pin holes, hazy finishes, dry spray (stripes) and poor leveling. Add in high humidity levels and blushing can become an issue. It is time to put away the fast thinners and start using slower thinners or retarders.
When the temperature is high a couple of things happen. The elevated temperature causes the coating to dry abnormally fast. The faster the finish dries the less time there is for it to flow out. This results in an increase in orange peel and poor leveling of the coating. The overspray produced at the outside edges of the gun’s fan can’t completely melt into the quickly drying surface and will appear as rows of stripes on dark surfaces.
When you spray out some finish on a piece of wood you will see the bubbles appear on the surface and pop. These bubbles are formed by the atomizing air from the spray gun and the solvents present in the finish. Since the finish thickens as it dries and the increase in temperature accelerates the normal dry time there is less time for the bubbles to form, pop and the surface to heal; the result is pin holes on the surface. In areas where there is excess build and the bubbles get trapped below the surface the finish will appear hazy; this is most noticeable on dark colors along edges and the intersection of stiles and rails.
When both the temperature and humidity are high blushing can become a problem. The high temperature causes the solvents in the finish to evaporate very quickly. The rapid evaporation drops the temperature at the surface of the coating. When the moist air comes in contact with the cool surface, condensation forms and the water mixes into the finish. The finish is drying so rapidly that the droplets of water don’t have time to evaporate and they are trapped in the coating resulting in blushing.
Solvent Based Finishes
The solution to these problems is to simply use the right thinner for the current environmental condition. All thinners or reducers are a blend of solvents formulated to evaporate at different speeds. Faster thinners are generally used in colder temperatures when finishes take longer to dry and the slower thinners are used in warmer temperatures when finishes dry too quickly and produce the problems noted above. The slower evaporating solvents in these reducers will keep your finish wet longer so that it has time to flow out and level. The bubbles in the finish now have time to float to the surface and the surface is wet enough to heal after the bubbles pop. The slower evaporation rate also means that the temperature at the surface of the coating does not drop as much so less condensation is formed and any water that does get into the finish has time to evaporate back out.
While slow reducers can solve many problems they can also cause problems. The longer dry time will make finishes more susceptible to dust entrapment and sagging on vertical surfaces. Too much of a slow reducer can also raise the sheen. Be conservative when adding these slow solvents. Start with a little and then add more only if you need to. You can always put more in; you can’t take it back out.
Elevated temperatures are waterbornes friend, elevated humidity – not so much. Warmer temperatures usually mean faster dry time for waterborne users, as long as the humidity stays reasonable. There does come a point however when the temperature is so high that the surface of the coating starts to skin over preventing the water in the layer below it from evaporating. The result is that the finish looks a hazy or has a bluish cast. As the relative humidity rises the air becomes more saturated with water vapor and there is less room for it to accept the water that is evaporating from the finish. The result is slower dry times. The simple solution to both problems is to spray on a lighter coat. In high humidity situations once the finish has flashed off you can put the pieces in front of a fan to speed drying.