i normally use this white nylon thread. but im out. but my grandmother gave me this spool of this really thick thread that if u double thread, you cant even break with your hands. but i have skool, and i didnt get a chance to sew with it yet
i had some nylon synth. thread which was unbreakable. ran out of it. using waxed tooth silk (term?) now. works pretty good. never had a seam broken with one of those threads.
take the thread, double it, and wrap it once around your index finger. then put your thumb on the buttom, and roll your fingers (index going backwards, thumb going forwards) and you should end up with like a rolled up loop, then slide that loop up the thread and you get a nice big knot
Nylon kite string, its a pain but you separate the 3 small strings, and it is impossible to break a single one with your hands. Also, if you have trouble knotting polyester(or nylon), just burn/melt it, but be careful.