Masking edges is more common for sharpening techniques. Depending on your software, you want to select the edges as if you drew a pencil drawing on the edges, and used this drawing as a mask to avoid adding noise on thoses edges, while maintaining the texture given to the rest of the picture. One way to do this is to use the Filter/stylize/find edges. The result is best with a slight gaussian blur. The Filter/adjustments/threshold enables you to adjust the thickness of the drawing. If you use the result as a mask before applying noise, the edges will be exempt from noise. There are other ways based on highpass sharpening or minimum/maximum filters, and certainly others I don't know... I am sure others can point you to useful tutorials.