Can you use a number 18 needle and buttonhole thread together to topstitch a quilt?
Pretty much if you can get a thread through the eye of the needle without it shredding and breaking, you can quilt with it.
My concern, though, is that if the thread is very thick, that it won't actually go into the batting layer of the quilt sandwich when the stitch is formed, that the stitching will look more like it has been couched from below with the bobbin thread showing on the top. To minimize the look of the bobbin thread, just use the same color
as is in your needle.
The only sure way to know is to test with a scrap quilt sandwich made from the leftovers of your quilt—same fabrics, same batting, same backing—and then stitch in the same patterns that you intend to use on the quilt, itself.
I suspect, that due to the thickness of the buttonhole thread, you'll need to reduce your needle tension, too.
I'm curious what you decide. Do let me know.
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