Outside Corners & Back-Rolling
Lee G. Jones / March 2015
Q: While observing the gypsum board installation at a construction site, the installer explained to me that industry practice when installing gypsum board on an outside corner of a wall is to stop both boards short of the corner (in other words, not to lap one board over the other) so to allow the metal corner trim to maintain a straight edge. This seems like a bad practice because the corner would be hollow and unable to support the corner bead, but the contractor was adamant. I have read my copy of GA-216, Application and Finishing of Gypsum Panel products; ASTM C840, Standard Specification for Application and Finishing of Gypsum Board; and the drywall construction handbook, but I cannot find a description of this method of building outside corners in any of them. Does such a detail exist?
A: The assertion on the part of the gypsum board installer that this method is the industry practice may be a slight exaggeration. I scoured the above mentioned reference standards and a few others, and I found no mention of or any detail showing the edges of both boards approaching an outside corner yet not overlapping. Clearly, bringing the two boards together where they barely touch at the surface closest to the framing would leave a hollow space underneath a conventional piece of corner bead that would likely result in its crushing when inevitably impacted by a moving object with any mass, such as a suitcase, hand truck or tricycle.
One fact about relying on such standards is that more often than not they omit any mention of proprietary materials—I suppose that’s because it’s pretty much impossible to reach a consensus on a practice that involves using a product that only one of the participant’s offers. Remembering this fact, and seeing a discussion about trim pieces used with prefinished panels, led me to an Internet search of available trim products for use with gypsum panels. Sure enough, there are indeed products offered for outside corners that are purported to be robust enough to withstand the inherent abuse of their location that abut the ends of the gypsum panels. One thing worth mentioning about the versions I saw, however, is that they get attached to the framing before the installation of the gypsum panels, which overlap the legs/flanges of the trim.
Q: We’re experiencing a texture difference in the dried paint film where the paint has been applied over the bare gypsum board surface and the joint compound. The gypsum board supplier tells us that back-rolling the paint would have prevented this texture difference. How can you tell if paint applied over a gypsum board surface has been back-rolled?
A: Suggesting that back-rolling an unknown material would avoid any and all problems with a paint finish over a gypsum board surface seems a bit simplistic to me. Several missing elements need to be addressed before attempting to answer this.
Perhaps most important, what are the paint manufacturer’s recommendations for the application of their product? Is the paint suitable for direct application over gypsum board surfaces, or should it be applied over a primer? Can it be spray applied and back-rolled? If it can be spray applied, what is the recommended tip size for the spray gun? What is the recommended nap length for rolling and/or back-rolling? Does the paint dry with enough of a sheen/gloss to warrant a skim coat over the gypsum board surface before priming or painting? Other questions include these: What are the drying conditions of the job versus the recommendations of the paint system? How many coats of paint were specified, and how many coats does the manufacturer recommend for proper coverage? Is the paint a deep/dark color?
I realize that is a lot of information to digest, but there are many variables to weigh when trying to troubleshoot an appearance problem with paint over gypsum board. The one step that helps avoid many of these problems is the application of a skim coat because whatever the paint does over the surface, it does it fairly uniformly. Conversely, if the paint dries differently over the bare gypsum board versus over the treated areas, how do you decide which finish is the intended one or what when wrong and where?
Many things can happen that back rolling may or may not help. For instance, if the paint requires a particular primer between it and bare gypsum board, the method of application is not likely to replace a missing primer. If the drying conditions are not ideal, the wet paint film may sit longer than normal on the surface. And if there is no skim coat, it can soak into the facing of the gypsum board and sit on the surface of the treatment, which may result in a profile difference. Or it may be unusually warm and dry, which causes back-rolling to lift the paint from the surface unevenly. Or the back-rolling operation may have been done with the incorrect nap roller, which either delivered too much or not enough paint to the surface. And so on.
Often such calls require the presence of the paint manufacturer’s tech rep, and sometimes the tech rep will need to send samples and photos to the lab to figure out what happened. So, pinning the problem solely on a lack of back-rolling is likely a bit premature.
Lee G. Jones is AWCI’s director of technical services.