SewLibra:
For very thin knit I like to stabilize the hem with Pellon Knit-N-Stable Tape. It's soft, stretchy, and does not change the drape.
SewLibra:
The top pic is without the stabilizer and the bottom is with it. I did not get tunneling, but the edge is wavy. That would not look good!
SewLibra:
First press up your hem. I like to use a seam gauge and pins to make sure the hem is even all the way around. This ponte knit is beefy and did not need a stabilizer.
SewLibra:
First place your hem under the coverstich foot wrong side up. Place the left needle just to the right of the raw edge and lower your needles so both of them penetrate both layers of your hem. This will help avoid tunneling, or bunching, between the rows.
SewLibra:
Place a strip of painters tape to the right of the hem, carefully lining it up. Painters tape will not leave a sticky residue on your machine and you can use one strip multiple times.
SewLibra:
Now turn your hem right side up and sew, using your tape as a guide. It's easier to keep on an even keel if you slow down your speed!
SewLibra:
I use this inexpensive dental tool from the drug store for pulling forward my coverstitch threads. Works like a charm!
SewLibra:
On the outside there is no tunneling. The only setting I changed from recommended factory settings is the stitch length to 3.5. I have a Janome Coverpro 900CPX.