Sharon Schamber soaks her thread in mineral oil. Poly or cotton. If you don't know who Sharon Schamber is, she is the master quilter of the master quilters. Every quilt is pure perfection. I started soaking my thread in mineral oil years ago when she explained why she did it. It does not make the fabric greasy. In her classes she had mind blowing tips. Her way of blocking a quilt to make it be perfectly flat will make you go what???? She uses a pressure washer.