I'm not an expert, but I do strip my own terrier, and my sister's Westie. On the Westie, as she's just a pet, I use thinning shears to do the underside of the tail, as it's a tad painful for her. My Oliver, when I tried to strip his tail, it did the same thing, left a bald patch on the top, so I leave his tail feathery
I also just use my finger and thumb, no knife, as I find it easier on the dog. I do, however use Chalk for the Westie.
I commend you for stripping your dogs, I much prefer a stripped Terrier, clipping ruins their coat, imo