Best Practices:
Finishing Glass Mat Gypsum Board


Glass mat wallboard is a noncombustible interior panel with a moisture-resistant fiberglass-reinforced gypsum core and coated fiberglass mat face that increases the product's overall strength (see ASTM C1658/C1658M-13).

These treated cores and coated facings made with fiberglass provide greater mold and moisture resistance than conventional paper-faced gypsum board. That can be a significant advantage if moisture damage from ambient humidity, standing moisture, or construction issues is likely.

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

Coming Indoors

In the past, glass mat wallboard was used primarily as exterior sheathing for exterior wall assemblies because of its moisture resistance. Now, however, we see it specified more frequently 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 may also be the material of choice for fast-track construction projects that require installation of the wallboard 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.

Glass mat gypsum can withstand incidental wetting such as an unexpected downpour before the building has a roof and windows.

The Pain in Painting

Despite its advantages, interior glass mat gypsum board presents a challenge to the painter. Instead of painting paper, the painter is essentially painting a fuzzy, prickly, fiberglass surface.

The paint system must fill that 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 other areas—more so than with paper-faced board—so preventing joint telegraphing is a greater challenge.

joint telegraphing

Conventional paint systems won't hide joint telegraphing with glass mat gypsum board. Additional finishing steps are needed.

Conventional paint systems cannot hide such substantial differences in surface appearance, so additional steps must be taken.

An ASTM Level 2 Drywall Finish may suffice for glass mat wallboard surfaces that are not seen, or are in areas like garages and storerooms where aesthetics are not important. But for walls that need a uniform appearance, even the Level 4 finish often specified for conventional gypsum board will yield an unsuitable appearance unless considerable care (and additional expense) are undertaken.

Prime Advice

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% to 40% solids or more).

Few products found in MPI #50 meet this criteria. Remember that percent 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 on the surface. Higher-solids products leave 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.

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

ASTM Level 5 Drywall Finish

An ASTM Level 5 Drywall Finish, which requires a skim coat, will produce the smoothest finish on glass mat gypsum board.

The specified topcoats must also be suitable for the exposure environment and will require at least two coats. Again, higher-solids products (30% to 45%) are recommended.

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

The Solution: Level 5

The better solution is to 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 colors may offer an aesthetically acceptable surface.

So while an ASTM Level 4 Drywall Finish is often specified, it's recommended that the contractor request that the spec be changed to a Level 5 for glass mat gypsum board.

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.

Spec information for this article was provided by the Wall and Ceiling Institute's Wall & Ceilings Specification Standards Manual 2012.

This article was written by PQA Inspector Dave Lick and is reprinted with permission from the MPI (Master Painters Institute) newsletter. MPI content describes best practices for commercial, institutional, and light industrial painting.


Tagged categories: ASTM; Drywall; Good Technical Practice; Gypsum board; Interior Wall Coatings; Master Painters Institute (MPI); Painters; Primers

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