Sagging is a term used in painting. This is when gravity will cause an area overloaded with paint to fall or sink in a horizontal line. It happens with all paint applications; brush, roller and spray. It also occurs when painting overhead and in this circumstance it will appear as a drip. Strictly speaking, in application it occurs when too much paint is applied. The thickness of the paint application is measured in mm. Reference is also made to the application rate, which details the area in square feet of the amount of paint that can be applied to a surface at the recommended mm thickness. Of course, when sagging occurs it is when the specific area has exceeded the application thickness.
A sag most commonly occurs with spray applications. When you use an airless sprayer, a large volume of paint is applied to the surface you are painting. You should take special precautions not to over-apply the paint, especially around corners or cracks where the paint can build up very quickly. A basic entry level airless spray machine will spray a half gallon of paint per minute, a gallon can usually come close to painting an average sized room. It is a common practice when projecting roller back walls. This is to more securely apply the paint to the surface and also to remedy any warping that may have occurred. Falls can be seen more commonly in commercial applications, often in these circumstances the contractor has strict deadlines and the quality of the work is not the highest priority. When trying to achieve a spray coat, you can end up with a sag if you’re not careful. When using a sprayer, it’s best to have a sponge and 9″ roller in case the paint flakes off.
When rolling it is generally the same reason; excessive buildup of paint on the wall, but a completely different cause. While rolling, the paint begins to sink in before the roller hits the wall. The perpetrator here is removing the paint from the roller. The only way to do this is to back up in the pan. Those grooves in the pan are there for this reason. What they do is give you just the right amount of paint to roll a section by spreading the paint on your roller evenly. When training an employee, this will usually be one of the first mistakes when rolling. Warping can always be traced back to the way the roller was used. For example; sporadic or non-uniform paint movement will likely leave an area with too much paint, or at the top of the roll there may be a build up of paint which when it reaches the wall will cause sag. A back roller is always helpful to prevent this type of paint sinking from occurring. The best way to keep the paint from sagging is to roll backwards in the pan and roll the paint in a flowing “I” or “V” pattern. With the roller, you just have to keep in mind that the goal is to catch sag before it happens and to make sure the paint has been spread evenly without over-spreading.
Brush work is similar to rolling. It is similar in the sense that it is going through the same procedures. When applying paint to your brush, you need to use the right amount for the job. You need enough paint to obviously paint the wall, but not so much that the paint drips off the brush and onto the floor. On the other hand, someone wants the job done, so don’t dry brush. There are a few schools of thought on how to remove paint from a brush. One method is to shake the brush around in the paint container to remove excess paint. Another method is to pat or tap the brush against the “walls” of the paint container. Whereas I usually scrape down one side and then run the cut with a string at the bottom! However, all of the methods share the same goal of producing a workable amount of paint on your brush. As with the roller, planing is the same in that you should be brushing backwards. Anywhere along the brush stroke you can achieve a sag, your only defense is a back brush to spread the paint evenly and smoothly.
Another reason for a sinking is the condition of the environment. Usually this will not be the reason, however in extreme cases, and by understanding the properties of the paint, we can know why this might happen. When the paint dries, there are two parts to the paint, liquid and solid, while the liquid will evaporate and the solids will become the finished painting. On rare occasions, extreme cases of humidity and temperature can be the cause of paint sagging. With too much moisture in the air, the liquid will not evaporate, causing the weight of the paint to cause the paint to sag. An even coat can be applied and sagging can still occur as long as these symptoms are present. Careless painting on a glossy surface can also cause paint sags. Glossy surfaces are also slippery, so paint will slide across the surface and cause sinking. To remedy this situation a quick sanding is recommended. This will reduce the shiny effect of the surface and create a condition called a “tooth” that will allow the paint to “bite” and adhere to the surface more ideally.
My last interaction with a Sagittarius was in a commercial job. I was using my airless sprayer to coat twelve shelves in low light conditions. After applying the first coat, I was checking my work with a 500 watt work light and found a sag in the paint. I immediately went over the area again with a mini sponge roller and then sprayed on a light coat again to produce an even finish. If a subsidence is detected in time, you can back it up or brush it off. If it dries out, you should sand it with 60 to 100 grit sandpaper, depending on the severity of the buckling. If hand sanding doesn’t produce an even finish on the surface, you’ll need to plaster the area or even get out a belt sander. After surface preparation is complete, you will prime or apply a top coat.