Needle thread showing on the right side of garment

The dreaded showing of stitches when you look at the right side of the garments after you serged it.

You can tighten the tension on your left needle a bit that will minimize these issues, the best is to match the thread to your garment as close as possible.

Also if you press (not Iron) your seams after you sew to set the seam, this also helps to minimize the showing,

To completely keep needle thread from showing is to press the seams after construction and then, without stretching the seam, press the seam allowances over to one side and top stitch over that. You will have no needle threads showing through.

needle thread showing serger,serger thread showing on rightside of garment,