Sagging is a term used in painting. It is when gravity will cause an overloaded area of paint to droop or sag in a horizontal line. It occurs with all paint applications; brush, roller and spraying. It also occurs when painting over-head and in this circumstance will appear as a drip. Strictly speaking, on application it occurs when too much paint is applied. Paint application thickness is measured in mm. Also referred to is spread rate, which details the square footage area of how much paint can be applied to a surface at the recommended mm thickness. Of course when a sag occurs it is when the specific area has exceeded the application thickness.
A sag occurs most commonly with spray applications. When you are using an airless sprayer a high volume of paint is being applied to the surface you’re painting. You have to take special precautions that you are not over applying the paint, especially around corners or crevices where paint can accumulate very quickly. A basic entry level airless spray machine will spray half a gallon of paint a minute, a gallon can generally come close to painting an average sized room. It is common practice when spraying walls to back roll with a roller. This is to apply the paint more securely to the surface and also to remedy any sags that may have occurred. Sags can be observed most commonly on commercial applications, often in these circumstances the contractor is under strict deadlines and the quality of the job is not the top priority. When trying to achieve one coat with a sprayer you can end up with a sag if not careful. When using a sprayer it is best to have a sponge and 9″ roller with you in case a paint sag occurs.
When rolling its generally the same reason; over-accumulation of paint on the wall, but a whole different cause. While rolling, the paint sags start before the roller even touches the wall. The perpetrator here is getting the paint off the roller. The only way to do that is back roll in your 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 generally this will be one of there first mistake on rolling. The sag can always be traced back to the way in which the roller was used. For example; a sporadic or non-uniform painting motion will more than likely leave an area with to much paint, or at the top of the roll there may be some accumulation of paint that when gets on the wall will produce a sag. A back roll always comes in handy to prevent this type of paint sag from happening. The most adequate way to prevent paint sags from rolling is to back roll in your pan and roll the paint on in a fluent “I” or “V” shaped pattern. With rolling you just have to be aware, the goal is to catch the sag before it happens and ensure that the paint has been spread evenly without being over rolled.
Brush work is similar to rolling. It is similar in the sense that you are going through the same procedures. When applying paint on your brush you have to the right amount appropriate for the job. You need enough paint to obviously get paint on the wall, but not too much that the paint is going to drip from your brush land on the floor. On the other hand, someone wants the job done so please do not dry brush. There are a few schools of thought on how to get your paint off your brush. One method is to shake their brush inside the paint container to remove excess paint. Another method is to pat or tap your brush against the instead ‘walls’ of the paint container. While I generally scrape one side and then execute the cut with a underside bead! However all methods share the same goal of producing a feasible amount of paint on your brush. Just like rolling, brushing is the same in that you have to back brush. Anywhere along the stroke of the brush you can achieve a sag, your only defense is a back brush to spread the paint evenly and smoothly.
Another reason for a sag is the condition of the environment. Generally it will not be this reason however in extreme cases, and understanding the properties of paint we can know why this might happen. When paint dries there are two parts to the paint, liquid and solids, while the liquid will evaporate the solids become the finished paint. Rarely, extreme cases of moisture and temperatures may be the cause of a paint sag. With too much moisture in the air the liquid will not evaporate, causing the weight of the paint to produce a paint sag. A uniform coat can be applied and a sag can still take place providing that these symptoms are present. Careless painting over a glossy surface can also produce paint sags. Glossy surfaces are also slippery, so paint will then slip on the surface and cause a sag. To remedy this situation a quick scuff sanding is recommended. This will reduce the glossy affect of the surface and create a condition referred to as “tooth” allowing the paint to “bite” and adhere to the surface more ideally.
My last interaction with a sag was on a commercial job. I was using my airless sprayer to coat twelve shelves in a low light condition. After applying the first coat I was reviewing my work with a 500-watt work light and found a paint sag. Immediately, I back rolled the area with a mini sponge roller then re-sprayed a light coat to product a consistent finish. If a sag is caught in time you can back roll or brush it. If it dries you have to sand it with 60 to 100 grit sandpaper depending on the severity of the sag. If hand sanding will not produce a smooth finish on the surface you will need to plaster the area or even get out a belt-sander. After the surface prep is complete then you will prime and or top coat.