My sample of coverstitching and using some layers of Avalon has worked, so much better. Whilst I only ran the sample under some water and did not properly wash it the part where the Avalon was feels a little stiff compared to the other half of the piece which did not have the Avalon. At least that is encouraging and I will go ahead and 'doctor' my vest sleeves and fingers crossed nothing goes haywire.
I agree with what you say Ouryve about some webbing inside a hem on knit fabrics, but that is what Nancy Zieman was using, but I am not sure whether this was a wash away or not. I would have to go and sit through viewing her videos again to find that spot. By the way she had a fantastic iron which she was using on a small ironing pad right by her sewing machine, this iron to all appearances was just sitting flat on the pad, she lifted it up, used it and put it back down flat again and then suddenly the iron lifted itself up off the pad - some very special iron - never seen anything like this before.