When I use Superior 40 wt(K.Tut) or metallics, I have a 90/14 as recommended by them and its fine. But for 50 wt (eg Mettler) I use 80/12 or even 75/11 quilting. But if it appears "holey" it closes up after washing anyway. Sometimes if you have a 40 wt and a fine bobbin thread like Bottom Line with a 90/14 it does appear that there are holes, but the washing deals with it.