TM 1-1500-204-23-9 (1)      Aluminum   bare   metal   areas   will   be   alodine treated per MIL-C-5541, Classes 1 or 2, prior to applying the first primer. (2)  Bare magnesium metal areas will be dichromate treated  per  MIL-M-3171,  Type  6,  prior  to  applying  the first primer. NOTE Paint will not bond to the brown powdery surface of excessively treated magnesium   or   aluminum,   nor   will   paint bond to treated aluminum surfaces which are too dark. (3)      Steel    substrates    will    be    treated    with    zinc phosphate  per  TT-C-490,  Type  I,  prior  to  applying  the first primer. (4)    All  missing  or  deteriorated  skin  sealant  will  be replaced  using  MIL-S-81733,  Type  II,  Class  B,  sealing compound. (5)      Copper   and   copper   base   alloys   are   quite resistant  to  atmospheric  corrosion.    Protective  coatings are seldom required because of the inherent resistance of the metal.  Corrosion attack on such components will usually  be  limited  to  staining  and  tarnish.    Generally such change in surface conditions is not dangerous and should  ordinarily  have  no  effect  on  the  function  of  the part.    See  TM  55-1500-344-23.    Paint  finishes  may  be applied for decorative purposes. g.    Mating  Surfaces.    The  use  of  primers  on  similar and  dissimilar  metal  mating  surfaces  and  on  attaching parts is presented in the following paragraphs. (1)    Application  on  similar  metals.    All  seams  in which  the  mating  (faying)  surfaces  are  similar  metals will be protected by applying two coats of primer to each surface being joined.  Welded faying surfaces shall not be primed prior to assembly.  Faying surfaces that are to be    adhesively    bonded    will    be    surface    treated    and processed     as     specified     in     the     approved     bonding procedures prescribed for the assemblies concerned.  In addition to the required primer coatings, faying surfaces of   magnesium   alloys   will   be   filled   with   an   approved sealing compound. (2)  Application on dissimilar metals.  Each surface to be joined will receive a minimum to two coats of the appropriate    primer,    and,    in    addition,    the    following precautions will be taken: (a)    Where  magnesium  is  one  of  the  metals  of dissimilar   metal   faying   surfaces,   the   metals   will   be separated by use of an approved barrier tape.  The tape will extend not less than 1/4 inch beyond the joint edges to prevent moisture from bridging between the dissimilar metals.  Sealing compounds conforming to MIL-S-81733 be  used  as  moisture  barriers  in  lieu  of  tape  and  when used, the sealing compound will be applied between the surfaces  and  squeezed  out  of  all  boundaries  and  the excess  removed  in  a  manner  that  will  fillet  all  edges. The fillet width will be not less than 1/4 inch.  Joint areas which  would  retain  water  will  be  filled  with  a  sealing compound  conforming  to  MIL-S-81733  or  MIL-S-7124. When the use of a barrier tape or sealing compound is impractical  because  of  mechanical  or  other  factors,  a primed  5356  aluminum  alloy  shim  extending  1/4  inch will be used in lieu of the tape or sealing compound. (b) Butt joints consisting of a riveted magnesium sheet   and   a   riveted   dissimilar   metal   sheet   will   be protected  by  grooving  the  seam  to  a  width  of  not  less than    1/8    inch    and    filling    with    sealing    compound conforming with MIL-S-81733.  The depth of the groove will    be    sufficient    to    retain    the    sealing    compound conforming to MIL-S-81733, which will be subsequently applied    and    smoothed    flush    with    the    surfaces    of adjacent dissimilar metals. (c)    Butt  joints  consisting  of  a  magnesium  sheet and a dissimilar metal sheet, neither of which is secured by    mechanical    means    other    than    riveting,    will    be protected  by  use  of  an  approved  adhesive  barrier  tape or  a  primed  5052  aluminum  alloy  foil  which  covers  the cut  ends  of  either  or  both  dissimilar  metal  sheets  and extends not less than 1/4 inch beyond the joint edge. (d)  Organic adhesive barrier tapes or metal foils will not be used on exterior surfaces. (3)  Attaching parts.  Parts, such as nuts, bushings, spacers, washers, rivets, high shear rivets, screws, self- tapping  screws,  sleeves  for  shakeproof  fastener  studs, self-locking  nuts,  speed  nuts,  clamps,  and  the  like,  do not  need  to  be  painted  in  detail  except  when  dissimilar metals or wood contacts are involved in the parts being joined.  Such parts will receive a coat of epoxy primer, MIL-P-23377 (wet or dry, at installation).  Rivets (5056) in  magnesium  alloy  need  not  be  installed  with  epoxy primer.  For magnesium dissimilar combinations, follow the general requirements.  Close tolerance bolts passing through dissimilar metals will be coated before installation  with  a  zinc  chromate  compound  conforming to  MIL-C-11796,  Class  3,  MIL-  C-16173,  Grade  I,  or  a wet   epoxy   primer   in   accordance   with   MIL-P-23377. Aluminum alloy washers (5356) of suitable 9-9