Drywall 6 - Joint Compound, Mud and Tapes

To achieve a smooth finish, all screw and nail heads must be covered with mud and seams must be taped over. Joint Compound or Vinyl Spackling is known as mud. We prefer to use Vinyl Spackling because of its quick drying time of about one hour or more depending on the thickness of the application. (Allow longer for deeper cracks and crevices.) The conventional joint compound has a drying time of 8 to 24 hours depending on the thickness of application.

Work from a mud tray to keep any dried pieces and bits of debris out of the can. Cleanliness is very important. Keep the lid on the mud bucket airtight when you are not using it to keep it from drying out. Smooth the top of the mudflat and add 1/2 inch of water before sealing the can if it will be stored for some time.

The compound can be purchased in 1 or 5-gallon buckets. Unless you are doing a very small job, the 5-gallon bucket is a much better buy. While using, scrape the inside wall of the container often to keep residue from hardening and dropping pieces into the compound. Drywall Tape is available in paper tape, which is pre-creased and can be used on straight seams as well as corner taping. Self-adhesive fiberglass tape can also be purchased, which has the advantage of not needing a coating of mud underneath.

Application of Mud & Tape:

Cover all nail dimples, applying the mud flush with the panel. If you are using paper tape, apply the first layer of mud with a 4-inch mud knife. Apply enough mud to the seams for the drywall tape to adhere to and to cover the entire seam. Apply the tape and embed it into the mud. While the mud is still wet, apply a second layer of mud to cover the tape completely. At the same time draw your blade tightly over the surface to squeeze the mud out from underneath the tape so it is good and flat. Take care not to create any bubbles.

A popular alternative to paper tape is self-adhesive fiberglass or fabric tape. With this type of tape, an undercoat of spackling is unnecessary. Simply apply the tape over the seam, then add a layer of mud over the tape.

Inside corners must be covered with paper tape because the fabric tape will not fold. Again, apply a layer of mud first, covering each side of the corner, then embed the folded tape. Finish smoothing the mud over and around the corner using a special corner knife.

Outside corners require metal corner beads to provide stability and protection to the drywall. Cut it to fit the full height of the corner with tin snips and fasten it in place with nails or screws every six inches. Then mud over it like any other seam, feathering the spackling out from the corner.

This first coat of mud should be completely dry before you go on to the next. A heater with a fan will cut down the drying time considerably.

Use a knife blade to scrape off any dried chips at the seams and nail head areas.

Sand the first coat smooth with an orbital sander or sanding block using 80 to 100 grit sandpaper before applying the next coat of mud. You can wipe it down with a large wet sponge in lieu of sanding, but take care not to soak the paper or wash away the spackling.

The second coat of mud needs to be thinned down with a little water and applied with a 6-inch mud knife. Each coat you apply should be thinner than the last. Apply the third coat with a 10-inch mud knife for a smooth finish. Again, sand or sponge smooth after each coat dries. (Only two coats of spackling are necessary for textured finishes.)

Once the final coat has dried and been sanded, use a flashlight to check for blemishes. The light casts a shadow which makes it easier to see the irregularities. A good smooth finish is essential to every good paint or wallpaper job.

Always do a final sanding or sponging before preparing to paint or paper the newly applied drywall. A primer-sealer undercoat is necessary to seal drywall and unpainted plaster or wallboard compound. Priming also prevents the base wallboard and outline of mudding applications from "showing through" any paint or wall coverings you may apply. Many primer-sealers are also "sizing," a preferred prerequisite to wallpapering. We recommend an oil-based primer for bare drywall. But be sure to check with your dealer for special considerations with your chosen wallcovering. See our Painting Section for further details.