
Hi there, :-)
Firstly the line is called a dorsal stripe it is very common in young westies when the coat is coming in. I am sorry to disagree with F.A Brooks as it is a 100% total myth that a true westie always has it.. :-) sorry Fiona ;-)
This line down the coat is where the hair tends to be at it's harshest and where the coats changes first. You have to think about the origin of the westie how the breeds were mixed to get the white colours so it is natural to get some colour throw backs. Generally speak some of the harshest coats tend to be slightly off white. When you read some of the variations of the the breed standard over the years it says a coat with a slight colour but harsh is much preferable to a white coat is it is soft.
Some westies can still have it as an adult but a lot of youngsters grow out if it. It is believed that evening primrose can help. Once all the hard coat comes it it is less noticible even if it remains.
Generally when you strip them regularly it can appear.
Lucky for us :-) Fiona is a little off again ( sorry fiona ;-) in her guess about the good breeders all gone... westies NOW and Still do have some very very good breeders mixes of old and young. The top breeder in the UK and has been for the last 3 consecutive years is actually only 34 years and has had westies for 15 years. 2 or 3 of the top westie breeders in the UK are actually under 40. BOB at Crufts the last 2 years is a very good breeder in here early 40s
The learning of hand stripping is something that takes a bit of time. Chip away at it bit by bit and you will slowly get the hang of it. I generally advice if you can ask an exhibitor who is close to you to give you a hour lesson , perhaps it would cost you the price of a trim but a hour worth while.
Once you master the technique , I woudl suggest photocopy some of the pictures from books and pinning them up whist you are trimming. In valuable when you are not sure at a particular moment what to take off from where :-D