It is all in the tension. I use cotton because it is the easiest, rayon works with cotton really well and adds enough shimmer, but still the piece (FSL) has more bulk than rayon alone. Polyester Made from oil is the shiniest, stretchiest and needs a looser upper tension. There is also the combination (all purpose) with cotton exterior and polyester core. First made by Coats that is still the general sewing preferred! I have only attempted metallic in the upper thread with cotton bobbin Hoep this short synopsis helps. Live Laugh Download Stitch!