Five / Six inch Drywall Tapping Knife
A six inch taping knife, along with a drywall mud pan, is the most important tool a drywall finisher will use. It is used for cleaning out buckets, scooping drywall mud, coating nails and screws, bedding-in tape, cleaning off corner bead, coating angles, installing Strait-Flex®, mixing hot-mud, installing mesh tape, cleaning out coating boxes and countless other things.
Because of how vital a six inch knife is for a drywall tapper, most professionals choose to have at least two or three of this size drywall knife on hand when doing a job. Though six inch knives are more common than five inch knives, if you can get your hands on one, you will likely appreciate the advantages of a five inch knife over a six. There are, of course, smaller knives, four inch, one and a half inch, one inch etc, and these all serve a purpose; however, a five, or more commonly a six, inch knife is generally the knife of choice for general purpose use.
The sides of a five / six inch knife should curve gently from the top edge down to the handle. Unlike larger eight, ten, twelve or larger knives, a six inch knife should not have square corners coming from the handle before returning toward the top edge. In other words, it should be shaped like a half circle rather than a rectangle, or more accurately a half ellipse rather than an isosceles trapezoid. The reason for this shape originally started with the need to coat inside drywall corners, or inside angles. The curved edge helps to prevent gouging into the opposing side of the angle when coating one side of the angle.
What should you look for when buying a six inch knife? Generally the most important factor to consider with six inch knives is their flexibility. An experienced taper will prefer a certain flexibility when using the knife. A stiff knife is better for scraping material. A more flexible knife may be easier to use when coating angles and bedding in tape however, it requires using more pressure to press the mud from the back of paper tape. This is something that a only an experienced drywall finisher will get a feel for, or develop a touch for. Some drywall finishers prefer stainless steel knifes but these are generally stiffer and may be considered too stiff for some tradesmen.
One feature that is nice to have, though not essential, for a six inch knife is a metal tip on the bottom of the handle. This is can be used to hammer-in nails that are sticking out slightly from the drywall paper or to create a small indention in the drywall paper which can be useful for covering imperfections.
When buying a five / six inch drywall knife, it is good to have a couple of options available to choose from. As you get more familiar with the slight differences in size and flexibility, you will come to develop a feel for your favorite type of knife.