Q: When finishing gypsum panels, which size tool should be used with each level of finish?
A: The concept of levels of finish has been in place for many years. It was first co-developed by the Gypsum Association and the Association of the Wall and Ceiling Industry to manage expectations and provide a communication tool between owners, architects and contractors on the acceptability of finished gypsum panel joints. The architect could specify a level of finish based on a given design and project conditions that a contractor could meet. There are five levels of finish with the higher the number, the greater the probability of providing the allusion of flatness over the gypsum panel surface. The specification for levels of finish can be found in the Gypsum Association’s document GA 214, Recommended Levels of Finish. They are also referenced in ASTM C840, Standard Specification for Application and Finishing of Gypsum Board.
The first level starts with filling the recessed joint created by two adjacent tapered edge gypsum panels with joint tape and compound. The specification increases with each higher level, the number of additional coats of joint finishing material that is to be applied. This is done while decreasing the acceptance of “tool marks” in the finish and increases the width of the joint finishing material beyond the centerline of the gypsum panel joint. The wider that the finished joint is, the greater the probability of creating that allusion of flatness. To enhance smoothness, increasing the width of the tools with each succeeding coat of joint compound is suggested. Neither the Gypsum Association nor ASTM C840 dictate the tool size. However, ASTM C840 specification does give a recommended joint width. In Section 24.4.1 the specification states, “Finishing compound and all-purpose compound shall be applied with tools of sufficient width to extend a minimum of 3 1/2 inches beyond both sides of the center of the joint tape.” The choice of the tool is left up to the craftsperson.
The tools that are used to finish the joints are commonly called drywall knives. Looking at the literature of one prominent tool maker, the knives come in a range of widths from 4 to 24 inches. That provides the craftsperson with a lot of choices to consider. An experienced joint finisher was once quoted as saying that one practice for the inexperienced trades person to overcome is the tendency to “overwork the wall.” That is to not continually go over the same spot in an effort to achieve that ideal smooth appearance. This translates to use a broader knife as the craftsperson “feathers” the joint compound further away from the centerline of the panel joint.
Prior to the levels of finish being published, joint finishing was considered a three-coat process. The first coat would fill the gypsum panel taper and embed the joint tape. A second coat would extend beyond the first coat to achieve a total joint width of 7 to 8 inches. The third coat would then extend beyond the second coat by a minimum of 2 inches. After a review of the literature from National Gypsum Company and USG and a discussion with their respective technical staff the following is suggested: Use a 5- or 6-inch knife for the first coat, an 8-inch knife for the second, and a 10- or 12-inch knife for the third coat.
Applying that logic to the levels of finish is then straightforward. Levels one and two are similar to the first coat that is described above. Therefore, a 5-inch knife could be an appropriate choice. Level three then equates to the second coat, and an 8-knife could be considered. Level four is the traditional third coat, and this is where a 10-inch knife could be the choice. Level five is the highest achievable level and the entire surface of the gypsum panels are covered with a skim coat of joint compound. Many specialty products have been formulated for this application, but the original concept was for the application of joint compound. This is where the broader knives are used.
Apprenticeship programs and trade schools train the aspiring professional taper and finisher. Their programs may differ somewhat from what is suggested above. The overall intent is to minimize the time spent working the wall. The more the wall is worked, the higher the probability that the result will be unsatisfactory with the development of ridges and tool marks. Building product and tool manufacturers can provide further assistance, as well as associations such as The Drywall Finishing Council and the Gypsum Association. They are valuable resources to learn more about achieving the desired level of finish.
Robert Grupe is AWCI’s director of technical services. Send your questions to [email protected], or call him directly at (703) 538.1611.
