WonderFil InvisaFil Thread 2-ply cottonized polyester 100wt 2500 m / 2734 yards. InvisaFil polyester is a remarkable silk-like 100wt, 2-ply cottonized polyester. This ultra-fine but surprisingly strong thread has cotton-like characteristics and negligible stretch while also feeling soft in the hand. InvisaFil makes for super low bulk stitches and virtually disappears, making it the ultimate choice for English paper piecing, hand and machine applique, delicate lace designs, fine fabrics, creating texture when micro quilting, or whenever you don not want your stitching to show.