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Agreed...#24 is much better than #18. Braided is critical!A few years ago, not having easy access to “real“ cave line, I bought the closest thing I could find at Home Depot. Mine was yellow, but it looks very similar to this:
https://www.homedepot.com/p/Everbilt-18-x-250-ft-Nylon-Braided-Mason-Twine-Pink-73296/206094361
It worked OK, except for one thing: the line was quite slippery. It was very easy for knots to become loose. I lost a couple of things including a $100 light because the knot came undone. To stop that, I had to get in the habit of using a drop of super glue on the knot: just melting and mushrooming the knot did not work for long . In addition, the relatively light #18 weight meant it frayed relatively easily as well, so frequently used equipment had to have the line replaced from time to time.
I ended up buying a 1 pound spool of #24 dive line from Piranha Dive Manufacturing. The finish on the line was not nearly as slippery, and so far I have not had a problem with the knots loosening or fraying. I haven’t been using it for an extended period, so in my mind the jury is still out about whether this is a 100% solution or not, but I do know that it has been an improvement. If I had no line at all, I would try to skip the HD line and get line more like the Piranha line.
The big difference it, that normal nylon is usually buoyant. Cave line is negatively buoyant.
If you just tie a boltsnap it doesn't really matter which u choose..
If you need to put a line, or use a spool with a dsmb. A negatively buoyant cave line is better
He may be thinking of polyester....all nylon has a specific gravity >1.025, it sinks even in sea water. It may not sink rapidly, but there is no nylon polymer that I am aware of with a specific gravity <1.025. It could exist, but as a textile engineer I have never encountered it, certainly not in any of the "normal" variants.