 The use of a compressor allows this to be applied to walls as well as ceilings.When knocking this down, the mud is allowed to dry for a short period, then skimmed with a knockdown knife – a large, usually plastic (to reduce noticeable edges) knife.Knockdown texture reduces construction costs because it conceals imperfections in the drywall that normally require higher more expensive stages of sand and prime for drywall installers.