Drywall tape is a key factor in installing and repairing drywall—no question about it. Not only does it smooth over the seams in drywall joints, it covers dents, holes, and cracks in both finished and unfinished walls. But it is in covering the drywall seams, and it is the type of drywall tape used in these applications that seems to create the most controversy among drywall installers: the better tape to use, fiberglass or paper?
How Drywall Paper Works
To properly choose which of these papers to use in your drywall installations and repairs, you'll need to know about the purpose these tapes serve. Fiberglass mesh tape was intended to be used under veneer gypsum plaster. For this purpose, fiberglass tape works very well because of the veneer plaster's toughness.
As veneer plaster is applied to the fiberglass mesh tape, it penetrates through the mesh and creates a bond with the drywall board strengthening the bridge across the seam. By contrast, joint compound is not as tough as plaster. When it is applied, rather than veneer gypsum plaster, beneath the tape and to the drywall board, it doesn't work as well. The weakness of this application is that some topping coats are too thin and fail to reinforce the bond created by the joint compound.
Drywall Paper Installation Problems
Using fiberglass mesh tape is faster than using a joint compound with paper tape because fiberglass does not require a compound bed before it is applied to seams. For this reason, installers sometimes prefer the fiberglass option. Problems arise when the installer lays two coats of mud over the fiberglass tape, mistakenly thinking this will be strong enough to hold the bond.
Fiberglass Mesh Tape Disadvantages
When coated with drywall mud, fiberglass tape is not as rigid or tough as paper tape is with that same mud. And, it doesn't bond as well to flat surfaces. Consequently, this type of joint sometimes develops a crack from shifting the wall on which the drywall is installed.
Best Use for Fiberglass and Veneer Plaster
The use of fiberglass self-adhering tape and veneer plaster works better when installing it on cracks repaired on older plaster. In that application, two coats of plaster provide enough rigidity and strength to secure the crack and keep the tape in place.
Applying Fiberglass Tape Over Cracks
To successfully use fiberglass mesh tape over existing cracks, the crack is first cleaned, then the mesh tape is cut into pieces about 3 inches long. Rather than being placed lengthwise along the crack, they are bedded in drywall mud on each side of the crack. Then they are placed horizontally across the crack. It would help if you took care to avoid overlapping the tape edges because of the uneven surface this will create.
When this application has dried, the installer then glues drywall paper tape vertically over the crack and the fiberglass tape, using joint compound to glue it. Then, to hide the tape, additional coats of mud are applied. This application will usually create a slight rise in the surface after the mud has been applied. But you can even this out by working over it with mud.