Use #16 needles and upholstery thread (comes in white or blue at my local Wal-Mart) or any heavy 100% polyester thread. Cotton will rot. Blended cotton will rot. It takes my (2) spools of 225yd thread to sew a sail. Buy three, nobody likes going back to Wal-Mart. Polytarp has different properties from regular fabric. I have to crank my thread tension way up to 9 or I end up with rat's nests on the back side of the sail. I suppose you could over-tension if your machine went to 11 or something. But I can't so I don't know what that looks like. I just know under-tension makes for rat's nests on the back side of the sail. I use the widest possible zig-zag stitch for all the stitching on my sails.