Drywall professionals have been using the same techniques to finish drywall since it was first introduced in the early 1900s. Their materials are also essentially the same, although some have been refined. Fiberglass mesh tape is one refinement, introduced as a stronger alternative to paper, and hot mud, a strong and fast-setting alternative to conventional joint compound, is another. Understanding the characteristics and application methods of the materials you use will ensure a quality job.
Paper vs. Fiberglass Mesh Tape
Fiberglass mesh tape has two distinct advantages over paper: it is stronger and it doesn't lift or bubble. Moreover, some types come pre-glued, making the preliminary task of sticking them to the wall effortless. Despite these advantages, many drywallers continue to use paper because it is all too easy to catch the corner of your knife on the mesh structure of fiberglass tape and dislodge it. Paper tape is strong enough for most joints and works best if you moisten it before applying it. Choose fiberglass, however, for problem joints where cracking is a problem and for patching holes.
Joint Compound, or Mud
Joint compound is a liquefied gypsum product resembling mud -- hence the nickname -- and comes in several varieties, either in powder form or pre-mixed. Taping compound is heavier than topping compound, but an all-purpose variety is available that is suitable for both jobs. In addition, hardware stores stock hot mud, a type that sets to a harder consistency than the conventional variety. Hot mud can speed up your job, and is best for problem joints and wide gaps, but it must be used with care because it difficult to sand. Many drywallers opt for all-purpose compound for an entire finishing job.
Before applying paper or non-adhesive mesh tape, you must cover the joint with a thin layer of joint compound to hold it down. Especially when using paper, it's important that this layer be continuous and free of voids, or the tape will bubble or lift after the mud dries. The only way to repair these defects is to cut off the lifted sections and re-tape over them, which can be a time-consuming procedure. Apply the mud with a 4-inch knife, lay on the tape, then scrape it flat immediately. Keep your knife clean to avoid dust and dried mud from leaving defects.
After applying tape and giving the mud time to dry, professionals use a succession of wider knives to topcoat joints with several layers of mud topping compound. In this way, they gradually feather the edges to flatten the seam and blend it with the surface of the wall. When properly done, there is no need to sand an existing coat before laying on the next one. It is easier and more effective to scrape small bumps and defects flat with a knife and then coat over them. The final coat should be sanded, however, prior to priming and painting.