old water spot on sheetrock

 I want to use Kiln on the sheetrock where an old (now dry) water spot
was.  It's brown and slightly black spots in a small area of the
sheetrock.   I was thinking to either apply 2 or 3 coats of kiln
before the correct matching paint or perhaps a little bleach on the
spot and allowed to dry before applying kiln.  I might also lightly
sand the wall before either method.  Which method do you think I
should consider to do?