Gypsum The sequence of operations in three-coat gypsum plastering is as follows: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Lime Install  the  plaster  base. Attach  the  grounds. Apply the scratch coat approximately 3/16 inch thick. Before the scratch coat sets, rake and cross rake. Allow the scratch coat to set firm and hard. Apply plaster screeds (if required). Apply the brown coat to a depth of the screeds. Using the screeds as guides, straighten the surface with a rod. Fill in any hollows and rod again. Level and compact the surface with a darby; then rake and cross rake to receive the finish coat. Define angles sharply with an angle float and a featheredge. Trim back the plaster around the grounds so the finish coat can be applied flush with the grounds. The steps for lime base coat work are similar to those  for  gypsum  work  except  that,  for  lime,  an additional floating is required the day after the brown coat is applied. This extra floating is required to increase the density of the slab and to fill in any cracks that may have  developed  because  of  shrinkage  of  the  plaster.  A wood float with one or two nails protruding 1/8 inch from the sole (called a devil’s float) is used for this purpose. Portland  Cement Portland cement plaster is actually cement mortar. It is usually applied in three coats, the steps being the same as those described for gypsum plaster. Minimum recommended  thicknesses  are  usually  3/8  inch  for  the scratch coat and brown coat, and 1/8 inch for the finish coat. Portland cement plaster should be moist-cured, similar to concrete. The best procedure is fog-spray curing. The scratch coat and the brown coat should both be  fog-sprayed  cured  for  48  hours.  The  finish  coat should not be applied for at least 7 days after the brown coat. It too should be spray-cured for 48 hours. FINISH COAT APPLICATION Interior  plaster  can  be  finished  by  troweling, floating,  or  spraying.  Troweling  makes  a  smooth  finish; floating or spraying makes a finish of a desired surface texture. Smooth Finish Finish plaster made of gypsum gauging plaster and lime putty (called white coat or putty coat) is the most widely  used  material  for  smooth  finish  coats.  A  putty coat is usually applied by a team of two or more persons. The steps are as follows: 1. 2. 3. 4. 5. 6. 7. 8. One person applies plaster at the angles. Another  person  follows  immediately,  straight- ening the angles with a rod or featheredge. The  remaining  surface  is  covered  with  a  skim coat of plaster. Pressure on the trowel must be sufficient to force the material into the rough surface of the base coat to ensure a good bond. The  surface  is  immediately  doubled  back  to bring the finish coat to final thickness. All  angles  are  floated,  with  additional  plaster added if required to fill hollows. The remaining surface is floated, and all hollows filled. This operation is called drawing up. The hollows being filled are called cat faces. The surface is allowed to draw for a few minutes. As the plaster begins to set, the surface-water glaze disappears and the surface becomes dull. At  this  point,  troweling  should  begin.  The plasterer holds the water brush in one hand and the trowel in the other, so troweling can be done immediately after water is brushed on. Water  is  brushed  on  lightly,  and  the  entire surface is rapidly troweled with enough pressure to  compact  the  finish  coat  fully.  The  troweling operation is repeated until the plaster has set. The sequence of steps for trowel finishes for other types of finish plasters is about the same. Gypsum-finish plaster requires less troweling than white-coat plaster. Regular Keene’s cement requires longer troweling, but quick-setting  Keene’s  cement  requires  less.  Preliminary finishing of portland cement-sand is done with a wood 7-15