I bought the 20 mm for Zada who is a silk coated and Azrael who is a cotton coat, and it works beautifully on both. The secret is to find one that gives very easily when you push on the pens. A lady I met at the dog show who shows Yorkies has this brush and told me about it.
http://www.sitstay.com/cgi-bin/sdisp.cgi/HBCC35.lg.jpg