Repainted with other than heat-resistant paint. Therefore,   before   you   assume   the   refractory requires re-working. check the following: Condition of the tadpole gasket. The  condition  of  the  insulating  cement protecting the tadpole gasket. Horizontal  baffle  tile  for  large  cracks,  breaks, chipped corners, and so forth. Cracks in the castable refractory at the ends of the baffle tile. Tightness of door bolts. Air line to the sight tube to ensure it is clear and all  connections  are  tight.  If  necessary,  blow  it clear with an air hose. It is normal for refractories exposed to hot gases to develop   thin   "hairline"   cracks.   This   by   no   means indicates   improper   design   or   workmanship.   Since refractory  materials  expand  and  contract  to  some degree  with  changes  in  temperature,  they  should  be expected to show minor cracks because of contraction when   examined   at   low   temperature.   Cracks   up   to approximately  one-eighth  of  an  inch  across  may  be expected to close at high temperature. If there are any cracks  that  are  relatively  large  (1/8-inch  to  1/4-inch width),   clean   and   fill   them   with   high-temperature bonding  mortar.  Any  gap  that  shows  between  the castable  refractory  and  the  baffle  tile  should  be filled-in in a similar fashion. After  opening  the  rear  door,  clean  off  the  flange surface of the door with a scraper or wire brush. Clean the surface of the refractory carefully with a fiber brush to  avoid  damaging  the  surface.  Clean  the  mating surfaces of the baffle tile and the boiler shell. Remove all  dried-out  sealing  material.  Wash-coat  the  lower half of the rear door refractory before closing it. The upper half of the door contains a lightweight insulating material similar to that used in the inner door. A thin washcoat  mixture  applied  gently  with  a  brush  is helpful in maintaining a hard surface. The  front  inner  door  is  lined  with  a  lightweight castable   insulation   material.   Thin   "hairline"   cracks may  develop  after  a  period  of  time;  however,  these cracks  generally  tend  to  close  because  of  expansion when the boiler is fired. Here, again, a thin washcoat mixture is helpful in maintaining a hard surface. Minor repairs  can  be  accomplished  by  enlarging  or  cutting out affected areas, making certain they are clean, and then patching as required. Should the entire installation require replacement, remove existing material and clean to the bare metal. Inspect  the  retaining  pins  and  replace  if  necessary. Reinforcing wire suitably attached may also be used. The  recommended  insulation  is  known  as  Vee  Block Mix  and  is  available  in  50-pound  bags.  Mix  the material with water to a troweling consistency. Mixing should  be  completely  uniform  with  no  portion  either wetter or drier than another. Trowel this mixture into any areas that are being patched. If replacing complete insulation, begin at the bottom of the door and apply the  mixture  to  a  thickness  equal  to  the  protecting shroud. With a trowel, apply the mixture horizontally back  and  forth  across  the  door  in  layers  until  the required  thickness  is  reached.  Allow  the  mixture  to air-dry  as  long  as  possible.  If  immediate  use  of  the boiler is required, fire as slowly as possible to avoid rapid drying of the material. Whenever  the  front  or  rear  door  is  opened  for inspection,  the  head  gasket  should  be  checked  for hardening and brittleness. Doubtful gaskets should be replaced.  Coat  the  gasket  with  an  oil  and  graphite mixture  before  closing  the  door.  Make  certain  all gaskets retaining rivets are in place. The flange of the door should be clean and free of any hardened cement, scale,  and  so  forth.  Check  the  condition  of  the  rope gasket used as a baffle seal. Replace if necessary. If the rope  is  in  good  condition,  liberally  coat  it  with  an insulating pulp before closing. Make sure the rope is properly positioned. If it is necessary to replace the rope, wire brush the tube  sheet  area  to  remove  all  of  the  old  sealing material. Place a new piece of 1 1/2-inch-diameter rope gasket on the lip of the baffle tile. Hold it in place with furnace cement or an adhesive. NOTE Earlier  models  have  several  steel  bar segments   tack-welded   across   the   tube sheet  to  serve  as  a  gasket  retainer  for 5/8-inch-diameter  rope.  It  is  suggested that  these  bars  are  removed  and  1 1/2-inch-diameter  rope  be  used. Generously  apply  a  seal,  consisting  of  a  pulp mixture  of  insulating  cement  and  water,  around  the entire rear door circumference. Place the pulp around the  inside  diameter  of  the  head  gasket,  as  shown  in figure 2-16. Also coat the tube sheet area adjacent to the  baffle  tile.  When  the  door  is  closed,  the  pulp compresses to protect the tadpole gasket and to form a 2-18