Glass Mat Finishing

By: Terry Kastner

Glass mat products have proven, over the years, to be an exterior sheathing product far superior to those with a paper facing. Building on that success and responding to the demands from the architectural community to provide an interior gypsum product that is impervious to the effects of moisture, mold, and mildew, glass mat products have now been developed for interior use. Although these products perform very well when subjected to limited amounts of moisture, they present certain challenges when it comes to finishing.

The challenge seems to be in achieving a smooth transition from areas that have received joint compound, to that of the glass mat facing material. On standard paper-faced drywall products, sanding the joint compound to create a smooth transition is relatively straightforward; in fact, most manufacturer’s caution against over sanding. This appears to be just the opposite when it comes to glass mat products. Even though the manufacturers’ literature states that the glass mat products can be finished similar to paper-faced drywall, they stress the fact that these products must be “adequately sanded” to produce a smooth transition.

Due to the extraordinarily sturdy nature of the fiberglass mat, coupled with its rough texture, it is difficult to achieve that “smooth transition.” This fact becomes most apparent when the specifications have called for a level four finish. Even under non-critical lighting conditions and when using flat or low-sheen paints, the transition between compound and facing material is more discernable than on paper-faced products.

This leads to the question of whether or not a level four finish is appropriate, or should glass mat products be specified to receive a level five finish only? The NWCB believes that a satisfactory finish can only be achieved by application of a level five finish, but this raises a question on which method is most appropriate for achieving an acceptable level five finish.

In general, the manufacturers’ literature states that these products should be “finished in accordance with the most current version of GA-214,” which recommends a skim coat of joint compound or proprietary products specifically designed to achieve a level five finish. Although the proprietary products perform very well on paper-faced wallboard, they were not specifically designed for use on non-paper-faced materials. Under critical lighting conditions and with high sheen or dark coloured paints the areas that have received joint compound have a tendency to flash or telegraph, as if only a level four finish had been applied.

To prevent this telegraphing, the NWCB recommends, and the manufacturers’ technical literature seems to recommend, that the best method for achieving a level five is by “skim coating the gypsum board surfaces.” The skim coat method has the advantage of using joint compound to fill small imperfections and, when sanded, the light-fill provided by the joint compound results in a non-discernable transition and an acceptable level five finish.

Regardless of the specified level of finish or whether the drywall is a glass mat or paper-faced drywall product, the NWCB recommends that, before starting the finish work, construct a mock-up, in a sample-space room, demonstrating the specified surface appearance (level of finish and paint decoration) subjected to the lighting conditions under occupied conditions.

The punch list process can often times become one of contentious debate, especially when it involves judging the final decoration of gypsum wall board. Having an approved mock-up for comparison to the actual finished product can serve to eliminate or diminish subjective objectivity and prevent costly rework or exhaustive discourse.