Sulky among others have thicker threads you might try. Embroidery thread size is 40 but 30 is thicker.
I have had a problem of loops when I use two threads at the same time...
HTH. Suzanne
Suzanne, make sure that you thread BOTH lengths of thread SIMULTANEOUSLY through your machines 'loopers' before placing them 'together' through the one needle. Also put the ONE reel in a small container (jug) on the floor, so that it has an additional 'way' to travel. Believe me, it does work very well. I have used a batting below the material which gives an additional 'indentation' of the threads and provides an adequate presentation. Let me know how you find it. Patricia
Jenne, try using a double thread in the machine's needle when sticthing your purchased sashiko design. Thread both threads simultaneously through the loopers. Let me know if that gave you the desired effect. Patricia