COAT: The American Sign Language (ASL) sign for "coat"

The sign for "coat" is made by forming both hands into the letter "A."  (Not an "S.")  Place both hands near your shoulders. Then move both hands downward and inward as if putting on a coat.

COAT:

If you mean "coat" as in "a layer of anything that covers a surface" (coat of paint) you could sign "PAINT":

Sample sentence: Do you have a rain coat / jacket?

Notes: 

See: JACKET

See: RAIN

See: PAINT

Optional / advanced reading:  Coat: "Animal coat"