TM 1-1500-204-23-9 should be taken that all of the pigment matter has been included in the mixture. (b) Mixing requirements for the two component     polyurethane     paint,     when     applying     by conventional  air  atomized  or  airless  spray,  are  to  mix four   parts   of   Component   A,   which   is   the   polyester portion,   to   one   part   of   Component   B,   which   is   the catalyst or isocyanate portion. After the two components are mixed, the total mixture is left to stand for at least 30 minutes to deaerate.  The mixture (batch) is  then  thinned  to  the  proper  viscosity  depending  upon the spray equipment.  The mix at the specified ratio will produce  a  pot  life  of  eight  hours.    Therefore,  no  more paint should be mixed than can be used in an eight hour shift or period. (c) MIL-C-46168   paint   must   be   thinned   to spraying viscosity, determined by the user and the type of spraying equipment to be used.  A suggested starting point is obtained by mixing approximately four volumes of  mixed  MIL-C-46168  paint  and  one  volume  of  MIL- T81772  thinner.    The  paint  must  be  thoroughly  mixed with    the    thinner    and    continuously    agitated    during application. (d) Where quantities permit, and to eliminate the   30   minute   deaeration   time   and   potential   waste, plural    component    spray    units    can    be    used    which automatically and proportionally mix the two components directly at the spray gun nozzle.  Since the two    components    are    kept    separate,    the    pot    life consideration is also eliminated. (e) The    polyurethane    paint    can    also    be applied  by  conventional,  airless,  or  electrostatic  spray equipment.    In  cases  where  this  type  of  equipment  is used,  the  viscosity  can  change.    The  applicator  should check  with  the  paint  company  and  equipment  supplier for the exact criteria for viscosity, thinning and the best application processes. (f) Whatever  spray  equipment  is  used,  it  is necessary  that  all  spray  guns,  lines  and  nozzles,  when applicable,  be  thoroughly  cleaned  after  use  to  prevent hardening of the coating system.  If the coating hardens in  the  system,  the  system  cannot  be  cleaned.    That portion   of   the   unit   in   which   the   paint   has   gelled   or hardened must be discarded. (g) MIL-C-46168 paint may be applied over a wide  range  of  temperatures.    Temperatures  from  50  to 90°F (10 to 32°C) are recommended for best results. NOTE MIL-C-46168 paint is a catalytic paint and will cure under most atmospheric conditions after application.  However, at very low temperature, curing is retarded. (h) Test   the   mixture,   pressure,   and   spray equipment    settings    by    applying    a    test    coating    to masking   paper   or   similar   smooth   surfaces   prior   to application   to   aircraft   or   components.      Adjust   paint and/or  equipment  until  proper  test  coating  is  achieved. Paint  and  equipment  are  then  ready  for  application  to the equipment or components. (2) Procedures for application of MIL-C-46168 paint to bare metal substrates.  The total paint system for the polyurethane  coating,  as  with  all  other  paint  systems, requires cleaning, a pretreatment coating, an anticorrosive    primer,    and    the    polyurethane    topcoat. This applies to painting of bare metal substrates. (a) Cleaning.    The  substrate  should  be thoroughly     cleaned     of     all     oil,     grease     and     other contaminants  by  appropriate  methods  cited  elsewhere herein or as specified in TT-C-490. (b) Pretreatments. Finishing documents,   e.g.,   MIL-STD-193,   MIL-STD-186,   MIL-T- 704,   etc.,   provide   appropriate   directions   for   surface preparation.    For  steel  substrates,  the  pretreatments  to use  are  zinc  phosphate  per  TT-C-490  Type  I  or  wash primer  conforming  to  DOD-P-1  5328.    For  aluminum substrates, the pretreatment is alodine, MIL-C-5541. (c) Anticorrosive primers. The anticorrosive  primers  to  be  used  with  the  MIL-C-46168 paint system are of the epoxy type.  The epoxy primer to   use   with   steel   substrates   is   MIL-P-52192   and   for aluminum  and  magnesium  substrates  is  MIL-P-23377, or    MIL-P-85582.        MIL-P-53022    may    be    used    on pretreated   ferrous   substrates   if   the   use   of   lead   or chromates is restricted. (d) Topcoat.      MIL-C-46168   coating   is applied at 1.8 to 3.0 mils dry film thickness in two coats. The first coat should be applied within one to four hours of the application of the primer.  The second coat may be   applied   within   a   few   minutes   of   the   first   coat   or anytime  thereafter.    The  total  thickness  of  the  topcoat should not exceed 3.0 mils.  There is no maximum time between  the  application  of  the  first  and  second  coats, ideally within two hours.  No surface preparation, except for cleaning, will be necessary as long as the surface 9-14