In  order  to  select  a  protective  coating  to  the  best  pos-
sible advantage  the  conditions  of  exposure  should  be  thor-
oughly understood  first ;  other  conditions,  such  as  the
character  of  the  surface,  and  number  of  coats  to  apply
should  follow.
The  exposure  of  dry  paint  surfaces  may  be  conveni-
ently divided  into  eight  classes  as  follows:

1.  Ordinary  interior  exposures.
2.  Ordinary  exterior  exposures.
3.  Extraordinary  interior  exposures.
4.  Extraordinary  exterior  exposures.
5.  Extraordinary  exposure  to  heat  (other  than  atmos-
pheric).
6.  Extraordinary  exposure  to  cold  (other  than  atmos-
pheric).
7.  Extraordinary  exposure  to  liquids   (other  than  at-
mospheric).
8.  Extraordinary  exposure  to  abrasion  (other  than  at-
mospheric).
No.  1.  Ordinary  interior  exposure  rarely  covers  a  va-
riation of  temperature  of  more  than  60  degrees  F.,  conse-
quently the  expansion  and  contraction  of  the  surface  met
with  in  this  class  of  exposure  is  so  small  that  it  has  very
little  effect  upon  an  ordinary  paint  properly  put  on  and
of  good  materials,  neither  does  moisture  and  its  rapid

36


evaporation  prevail,  so  that  here  we  have  a  condition
notable  for  its  simplicity.  Take,  for  instance,  several
small  sheets  of  tin  or  iron  with  clean,  bright,  dry  sur-
face, coat  them  with  coal  tar  varnish,  asphaltum  var-
nish, or,  in  fact,  any  cheap  paint,  and  when  thoroughly
dry  lay  them  aside  in  the  drawer  of  a  writing  desk ;  20
years  later  they  will  be  in  as  good  condition  as  the  day
they  were  stored  away.  The  sheets  of  metal,  even  with-
out paint,  laid  away,  in  like  manner  for  the  same  length
of  time,  will  also  be  found  to  be  in  excellent  condition.
