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:
Notes: