I once did some sewing repair to an awning and used "fish line", as I figured it was used mostly underwater conditions. I did it by hand, and to get a double type stitch like on a sewing machine, I would start out by making about 1/2 inch stitch, then come back about 1/4 inch on the under side, then back to 1/2 on the top again. I used a single line to do the sewing, and not a double line like regular sewing.
My awning had deteriorated at the top from the sun, so I cut it back about 12" and reused the plastic "welt strip" and sewed it up the full length that way. I used clothes pins to hold the welt strip in place while sewing. So I now had a 7 foot awning instead of an 8 foot.
Dr Quick