Nylon is stronger than polyester in the same thickness. In other words: Nylon 69 is stronger than polyester 69, but nylon 69 is slightly less strong than polyester 92. Polyester 69 will last longer in a boat cover sewed with nylon 69, but polyester 92 will last far longer in the same circumstances. Don't be confused with the thickness of the thread, thickness does not necessarily mean strength. Cotton thread 1and 1/2 times as thick as nylon 69 may be thicker in appearance, but has far less strength. As far as the topstitching thread......... Coats makes a specific thread called topstitching thread. It is specifically designed to look heavier than their regular thread for appearance only. It is not necessarily stronger because it is and looks heavier. For example: Coats also makes an orange thread specifically to topstitch bluejeans. It may look heavier, but has far less strength than other threads. In other words: thread manufacturers make certain threads for no other reason than how they look on a specific finished piece, not necessarily how strong they are. What confuses the issue even more is the fact that there are no real sizing standards as far as thread goes. For upholstery work, nylon 69 is the standard, for different looks, there are dozens of threads to present other looks. Polyester 92 and polyester 138 (basically double the size of 92 but the same strength) are used for boat covers. Polyester 138 requires a larger needle (150 in my case and as large as your machine allows in your case) Polyester thread stands up better to direct sunlight. In any other application, stick to nylon for strength as opposed to size. My point is this: use whatever thread gives you a strong seam, and topstitch or french seam with whatever looks good to your eye. Whatever you do...........do not use nylon monofilament thread. This stuff is very cheap, but it heats up and stretches as it goes through a sewing machine, (even at low speeds , but especially at high speeds) and fails quickly. Bass Tracker used this thread to sew all their pontoon seats for two years and all of them failed after 5 years. They may have improved this stuff over the years, but............once bitten, twice shy babe. I apologize , once again , for being so wordy, but I was on a roll. Keep asking questions, dude, you're helping lots of guys when you ask.
Last edited by DanTwoLakes; 01-31-2007 at 09:46 PM.