: Also, Dyson recommended plain dental floss for sewing. Is this a
: common thread for SOFs?

It is common enough in that it is available in any drugstore, and it is certainly strong enough for the job.

But for sewing canvas or heavy fabrics I prefer thread or twist. My choice for this job would be upholsterers thread or buttonhole twist. Or buttonhole thread.

By definition floss is a collection of fibers running in the same direction. About the longest floss of natural fibers would be the horsehairs on the bow for a violin, string bass, or cello. Synthetic fibers can be extruded in very long strands so you'll find very long pieces of dental floss. Its strength comes from each fiber running the whole length of the floss.

If you twist your floss you will form a yarn. The twisting action locks shorter fibers together and allows us to make use of animal hairs and plant fibers as well as synthetics. Twist your yarn a bit too much and it will kink. At this stage you can place 2, 3, 4 or more yarns together and they will twist around each other to form strings, threads, twines, and ropes.

With all the twisting even fairly short fibers can be locked together to form a very strong material of great length. The diameter of the original yarn used for sewing threads is pretty small. They generally use 2 strands to make the thread used for sergers--avoid this. 3 yarns go in most standard sewing threads, and they make twist with 4 yarns. If they start with heavier yarns they get heavier threads or ropes.
Just like fabrics are sold by their weight, threads are sold by weight and gauge.

Upholstery thread is sometimes actually a 4-strand twist. Otherwise it is simply a stronger and heavier thread. When it is a dark color it is used for sewing on buttons. In a light or natural color it is used for sewing carpet and upholstery fabrics. At a fabric store you'll find it with the threads.
Carpet thread and sail thread are thicker and stronger, but sold in fewer places. And more expensive.

Craft stores and online sites can sell you very heavy threads used for tentmaking and sewing shoe leather. Nice stuff for sewing heavy canvas. Harbor Freight has a sewing awl which uses this stuff. The awl is useless for our purposes, but the spools of replacement thread work great and the price is resonable. www.harborfreight.com
There are also heavyduty threads made from the same material as your fabric. The advantage of using something like that is that it shrinks about the same as the fabric you are sewing. Less puckering when it gets wet.

PGJ