Agggh, never mind. Just went back and re-read Maine Sail's article. He doesn't use butyl string. He rolls a bit of butyl tape into the shape of string.
1 demerit for Bruce for posting without engaging brain...
I've bought rolls of butyl from Defender (they're right down the road from me), Amazon, True Value (look in the window / screen door section), and even MaineSail sells the stuff (http://www.pbase.com/mainecruising/butyl_tape). I would recommend the grey as the black seems to me more sticky and harder to remove.
Thanks. I have a couple of rolls of the butyl tape from Maine Sail (great stuff!)
My vague recollection was that he recommended using butyl string under the heads of fasteners. Some of the fasteners I pulled off last weekend had a thin stripe of butyl under the heads. So I thought butyl string was a "thing".
As it turns out, it isn't. Maine Sail just recommends rolling a thin piece of butyl tape into the shape of string....