The Challenges of Finishing Interior Glass Mat Gypsum Board







Spec information provided by the WCI Wall and Ceiling Specification Standards Manual 2012 Edition; pictures compliments of Northwest Wall &Ceiling Bureau

Interior glass mat wallboard is a noncombustible interior panel that consists of a moisture resistant fiberglass reinforced gypsum core and coated fiberglass mat face, which increases the overall product strength (see ASTM C1658/C1658M-12). These treated cores and coated facings made with fiberglass provide greater resistance to mold and moisture than conventional paper-faced gypsum board, which can be a significant advantage if moisture damage from ambient humidity, standing moisture, or construction issues is likely to occur.

Glass mat wallboard is also a stronger, more durable facing material than paper-faced board, so it's less susceptible to structural failure or delamination.

In the past, glass mat wallboard was used primarily as an exterior sheathing product for exterior wall assemblies because of its resistance to moisture. Now, however, we see it specified more frequently as a wallboard for interior environments subject to extensive moisture or liquid exposure, such as hospital walls that endure constant cleaning and where the potential for mold growth is unacceptable.

Glass mat gypsum board may also be the material of choice for fast track construction projects that require wallboard to be installed before the building envelope is completely enclosed, especially in storm-prone or humid environments. Hanging paper-faced drywall before the building is dried in can be a gamble; no contractor wants to rip out damaged paper drywall, wasting time and money and upsetting the schedule. But glass mat gypsum board can withstand incidental wetting such as an unexpected downpour that wets walls installed before the building can be topped off with a roof and windows.

The Pain in Painting Glass Mat Gypsum Board







While interior glass mat gypsum board has advantages as a wall panel, it presents a new challenge to the painter. Now, instead of painting paper, the painter is essentially painting a fuzzy, prickly fiberglass surface - and the paint system must fill the glass mat surface and eliminate texture variations to provide an even finish. Also, joints and nail holes touched up with joint compound will have a significantly different texture than non-touched up areas - more so than is seen with paper-faced board - so preventing joint telegraphing is a greater challenge.

Conventional paint systems cannot hide such substantial differences in surface appearance, so additional steps must be taken to achieve a satisfactory finish.

For glass mat wallboard surfaces that are not seen or in areas where aesthetics are not important (e.g. garages and store rooms), an ASTM Level 2 Drywall finish may suffice. But for wall surfaces where a uniform appearance is desired, even the Level 4 drywall finish that 's specified so often for conventional gypsum board surfaces will yield an uneven and unsuitable appearance unless considerable care (and additional expense) are undertaken.

If a Level 4 finish is specified, using the proper primer is critical. Instead of specifying one coat of a typical drywall primer (such as products approved under MPI #50 ), care must be taken to specify higher solids primers -30-40% solids or more - and very few products found in MPI #50 meet this criteria. Remember that % solids is how paint manufacturers describe how much of what's in the can is solvent (for water-based paints, the 'solvent' is the water) that evaporates after the paint is applied, versus how much 'solids' (pigment + resin + additives) are left behind on the surface. Using higher solids products leaves a higher build to fill in the glass mat texture.

A higher cost alternative is to specify MPI #137, which is MPI's standard for waterbased stainblocking primers; more of these products meet the high solids criteria.

And regardless of which primer you choose, two coats may be required to alleviate texture variations where the board meets the filler.

The specified topcoats must be suitable for the exposure environment, and will require a minimum of two coats to create a smooth surface - and again, higher solids products (30-45%) are highly recommended.

All of this amounts to a minimum 4-coat process. And even with 4+ coats, the use of dark colors or gloss finishes is mighty risky business that's very likely to yield unsatisfactory results.

The Solution: A Level 5 Drywall Finish







The far more preferable solution is to always specify an ASTM Level 5 drywall finish for glass mat wallboard. Level 5 requires a 'skim coat': a thin coat of joint compound trowel-applied over the entire board surface and wiped down immediately to leave a tight film of joint compound. Alternatively, the board may be covered with a product manufactured for the specific purpose of developing a Level 5 finish on glass mat gypsum board.

Now, the board may be primed with one coat of conventional drywall primer such as products found under MPI #50 and finished with conventional interior latex - and even gloss finishes or deep tone colors may offer an aesthetically acceptable surface.

In summary: while too often an ASTM Level 4 Drywall Finish is what's provided in the specifications, it's highly recommended that the contractor request the spec be changed to a Level 5 when glass mat gypsum board is to be painted; the added time and expense of trying to achieve a satisfactory finish over this surface can far outweigh the incremental expense of upgrading to a Level 5 finish.

Click here for more information from the WCI — Wall & Ceilings Specification Standards Manual 2012 on glass mat gypsum board finishing levels.

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