To sign "a rash" what you do is you show the sign "red" and then you
use a "classifier 5" (CL:5) to show "a rash" on your arm
-- or wherever the rash is in real life. For example, if the rash is
on your neck or face then do the sign on your neck or face
accordingly.
Also, if you are talking about "rashes" a lot in the same
conversation you can (should) drop the sign for RED after the first
time thus reducing the number of movements in the sign and making it
more efficient.
RASH:






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American Sign Language (ASL) online at American Sign Language University ™
ASL resources by Lifeprint.com © Dr. William Vicars