I use a UHV thread that holds up for years in the sun, wind, rain & snow of Flagstaff, AZ. This thread along with some great sunbrella fabric is what I use to make deck furniture covers, pillows and awnings. It's a heavy fabric but is still flexible. Be sure to look into heavy duty needles when sewing with this stuff. PM me if you want to know the company I work with. The thread comes in large cones but well worth the investment and I've used in in working with nylon fabrics as well.