The Collaborative International Dictionary of English v.0.48:

Undercoat \Un"der*coat`\, n.
   1. A coat worn under another; a light coat, as distinguished
      from an overcoat, or a greatcoat.
      [1913 Webster]

   2. A growth of short hair or fur partially concealed by a
      longer growth; as, a dog's undercoat.
      [1913 Webster]

WordNet (r) 3.0 (2006):

undercoat
    n 1: seal consisting of a coating of a tar or rubberlike
         material on the underside of a motor vehicle to retard
         corrosion [syn: undercoat, underseal]
    2: the first or preliminary coat of paint or size applied to a
       surface [syn: flat coat, ground, primer, priming,
       primer coat, priming coat, undercoat]
    3: thick soft fur lying beneath the longer and coarser guard
       hair [syn: undercoat, underfur]
    v 1: cover with a primer; apply a primer to [syn: prime,
         ground, undercoat]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "undercoat":
   chromogen, coat, coat of paint, coating, color, color filter,
   color gelatin, colorant, coloring, dead-color, distemper, drier,
   dye, dyestuff, exterior paint, flat coat, flat wash, floor enamel,
   ground, interior paint, medium, opaque color, paint, pigment,
   prime coat, primer, priming, stain, tempera, thinner, tinction,
   tincture, transparent color, turpentine, turps, undercoating,
   vehicle, wash, wash coat