The Collaborative International Dictionary of English v.0.48:

Scratch coat \Scratch" coat`\
   The first coat in plastering; -- called also scratchwork.
   See Pricking-up.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pricking-up \Prick"ing-up\, n. (Arch.)
   The first coating of plaster in work of three coats upon
   laths. Its surface is scratched once to form a better key for
   the next coat. In the United States called scratch coat.
   --Brande & C.
   [1913 Webster]