"Undercut plasters" often used as a term to describe the process of undercutting plaster. This process involves using a sharp, round object such as a specially created tool or a metal lid and underscoring a groove into existing plaster or material. This allows new plaster to ooze into the groove and set in order to give the plaster a firmer grip. Undercutting is used as a technique to add depth and shape and to fill pre-existing holes in plaster.
When using plaster to sculpt a design, many artists find the need to add layers, additional details and even objects to their pre-existing bases. As an example an artist constructing a human face out of plaster would need to first cast a base to design the face from. Undercuts would then be made for the lips, nose, cheekbones and ears so the artist could mold and add these individual details. Using undercuts for this type of plastering allows somewhat stronger and more detailed work.
Because drywall or gypsum walls are weak, they often need patching or repairing. While it is possible to simply fill a hole with plaster and allow it to dry, this method often allows the plaster to shrink as it dries so that it no longer completely fills the hole. By undercutting behind the existing plaster, it is possible to allow the patch to ooze behind the wall so it is not only stronger but more appealing to the eye.
Because undercut plasters require that a groove be cut into pre-existing plaster, it can cause weaknesses. An improper cut that runs too deep can crack remaining plaster or create a fracture that widens with time. Multiple undercuts in one location can often cause a great deal of weakness that could result in structural collapse of a sculpture or fracturing and cracking of a wall. These problems can usually be avoided by strengthening walls and spreading undercuts over a wide area.