One can get 'buttonhole twist' which is quite a lot thicker than the standard stuff I'm using. I've experimented with both, I've found it's quite hard to get really thorough coverage of the gimp with the finer thread but perhaps I just need (even) more stitches!
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In my opinion, the thicker the thread, the better it is for buttonholes. I would always use the buttonhole twist, and never use the regular hand-sewing silk.
Also, if you can't get the silk, a polyester thread called 'Gutermann top-stitch' is very good for buttonholes.
If you can get Buttonhole twist ‘30 or ‘40 = thickness of thread.
Hi,
I've listed the Gütermann silk threads here for blue and grey: https://www.handcrafttailor.com/forum/general-discussions/european-mainland-getting-threads-and-tools
Gütermann R753 is a 40s silk thread, available in 60 colors.
Silk buttonhole twist is what's usually recommended. I also have bought Gütermann R753. Someone once told me that it was OK to practice with basting thread.