Wall Application

• The method of using wire mesh and a scratch coat layer of mortar has been used for years with wall applications and it is perfectly acceptable for creating a completely plumb and flat surface for the tile to adhere to. The mud is mixed fairly thick and lime is included in the mixture to help it stick to the wall while you work your way up and smooth it with a flat trowel and levels between screed sticks that help define the thickness.
  
  

Floor Applications

• Untrained tile installers who have earned their degree at local home improvement stores without completing an apprenticeship are sometimes taught that installing a wire mesh and scratch coat as a floor underlayment is an acceptable method, but it is not. This is absolutely not an approved method for preparing a ceramic tile floor and you will likely have failure within a year because there is no structural integrity to this method for floors.
  
  

For Ceilings

• Applying a scratch coat or skim coat to ceilings is a common process during the preparatory stages of a shower installation. The reason you coat the ceiling with a thin layer of mortar beforehand is so that you can “burn” the thinset into the ceiling and let it dry so that when you come back to install the actual tiles there is already a layer of thinset in place to help bond everything together and ensure that the tile never falls off the ceiling.
  
  

For Tile

• Scratch coats and skim coats are used on the backs of tiles to help ensure there is total coverage and bond. It’s not necessary for all types of tile, but you can choose to skim coat the backs of your tiles for extra protection in any installation. The only time it is a requirement is with ceiling tiles or with natural stones whose colors are affected by thinset mortar and you don’t want the trowel notch marks showing through the face, or with stones that require extra mud to make up for size variations.