I use "regular" weight thread for very small tight areas, but 99.9% of the time I use Mettler 30 weight (it has orange writing around the top). My industrial machines do a very tight satin stitch and it also helps make the heavy weight stiffener on the back fall off easily.
satin stitch showing on the back of the stiffener which is already peeling away.
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stiffener is gone
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front
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closeup of stitches which should be flat and smooth
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