Westies are highly prone to skin and ear troubles, especially as they get older. I believe it is dermatitis/eczema.
Our guy is nine years old now. A couple of years ago, he started to get the problem you described mainly along his spine, neck and tail areas. He always had trouble with ear infections/discharge, and that got worse, too.
We all but cured the skin problem by taking him off processed food completely and making his food ourselves. There was an occasion recently when we did not have time to make the food, so we bought him a good-quality kibbled food for sensitive skin. The skin problem came back, and was cured again within days of putting him back on our home-made food. The evidence points strongly to processed food being the cause of our westie's skin problem. Our home-made food is ground meat, rice, carrots, horseradish, green veg, wheat bran, yeast extract, agar jelly, and water. I can give you a document with the recipe if you give me an email address, Skype, or something.
We also bathe him at least once a week with a shampoo containing tea tree oil and a little olive oil. We mix that ourselves, too. About 250ml Cooper's all-purpose animal shampoo to 700ml water, 30ml of tea tree oil, and 10ml of olive oil.
He does still have a little eczema between his toes and pads. That is controlled by applying a smear of petroleum jelly (Vaseline) mixed with tea tree oil once or twice a week.
Our vet has prescribed every kind of medication you could name to treat his ears. Nothing works. I now control that by cleaning his ears thoroughly at least once a week after bathing. I use Nolvasan ear cleaning lotion, applying it three times then wiping with a twist of tissue to clean out all the discharge and any water that got in there during bathing. I always finish by drying the ear with a cotton bud (MUST BE DONE VERY CAREFULLY). The result is minimal ear irritation that should last for a about a week before they need cleaning again.