Why does my baker's twine keep snapping mid-stitch when I hand-sew burlap?

Baker's twine is the problem, not your technique. It's basially two thin decorative plies with almost no real strength, and jute burlap is abrasive enought to chew through it mid-stitch, expecially when the twine drags across raw burlap edges on every pass. The waxed version feels worse because the wax grips the fibers and makes the friction worse instead of letting it slide. If you want that rustic twine look, switch to waxed linen thread or even button thread in a color that reads similar, or go up to the thin jute twine used for garden tying, which is what it was made to do. Whatever you use, work with shorter lengths, maybe arm's length, so the thread doesn't get weakened by repeated abrasion before you finish a seam.

For the fraying I just ran a bead of regular white glue diluted with a little water along every cut edge and let it dry overnight. Stiffened everthing up and no more shedding on my floor. Did that on a jute table runner years ago and it held up fine after, so I trust it on a costume that only has to survive a festival day. Also on the needle, I'd swap the tapestry for a sharp one. The blunt tip makes you push harder and that's prolly part of why your twine is popping. Worked way better for me on coarse fabric anyway.

That fraying is nothing new, my mother sewed feed sacks into everything back in the day and she never owned a serger in her life. Her trick was to turn each cut edge under twice and catch-stitch it down flat before assembly, so every raw edge is allready locked into a fold before the garment gets any stress. Slow work but it holds. On the thread shredding, she always ran her thread over a lump of beeswax and pinched it in a tissue to set the wax, then threaded it fresh. Worth trying before you commit to buying anything fancy. And get yourself a proper thimble, your pushing finger will be raw by the third ruffle otherwise.

One trick from my days making prop costumes for a haunted house, about 40 yards of jute edging per season, was to twist two strands togehter before threading. Doubles the bulk, halves the snapping, and the twist actualy looks more period-appropriate on a rustic gown than skinny baker's twine anyway. Also cut your twine at least a hand's width longer than you think you need so you never strain pulling the last stitch tight. Out of curiosity, how are you anchoring the buttons? Sew-thru shank buttons on burlap can pull straight through the weave unless you back them with a scrap patch on the wrong side.

The glue edge trick is the way to go, I can back that up. I did the same on a burlap wreath base a few years back, except I ran the diluted glue along the edge and then pressed everything flat with an old credit card before it dried, which kept the fibers from curling up while they stiffened. Held through two winters hanging outside. One heads up though, wear an old long sleeve while you're handling that burlap, those little fibers find thier way into your skin and itch for hours.

Update: twisted two strands of twine together like the haunted house costume person said and the snapping basically stopped. Plus it does look better, chunkier stitches suit the whole rustic thing. For the fraying I went with the feed sack trick, turned every hem under twice and catch-stitched it flat. Skirt has survived three test laps around the living room with zero unraveling. Festival is saved.