Here is the biggest and most important way to ensure success when sewing knits:
TEST BEFORE YOU SEW
When you cut out your fabric, save 3 or 4 pieces of those hand-sized left-overs.
Set up your sewing machine with the needle and thread you think you’re going to use. Stitch a line on a single layer. Does it look alright? Congrats you’re good to go. But what if the holes gap open? Or there are skipped stitches? You need a different needle size. Holes too big, go down a size. Skipped stitches go up a size. Is the line of stitching laying flat or is it bunched, puckering or gathering. This is a little more difficult. Yes it could be the needle size. But if your holes closed nicely and you don’t have skipped stitches, I’d decrease the top thread tension. If you pull on opposite ends of the fabric, do the stitches pop? Try a narrow zig zag instead of a straight stitch. Repeat this for all the stitches you’ll be using on this project. I test my fix stitch, as well as any decorative stitches I may want to use. Repeat again with 2 layers of fabric and this time, press open the seam and see how it looks.
You can SUCCESSFULLY sew knits with just straight and zig zag stitches using a ball point needle. I’m a serger owner. I also test my serger with 1 and 2 layers of fabric. I press the serger seam to one side, press and hold it up to the light. I want to be sure that the tension on my serger needles are tight and I won’t have an unintentional gap down the seam line.
Now this sounds like it might use a lot of time. If this the first time you’ve done any testing it might take you awhile. But for me I’m pretty confident that my first guesses as to needle, stitch and stitches settings will be correct. Unless I’m using decorative stitches, I’m usually making only 4 6-inch seams (2 at the sewing machine, 2 at the serger; 1 layer and then 2 layers of fabric); and I’m done in less than 2 minutes. YES you will be doing the same after a few testing sessions.
But I said save 3-4 of the hand-sized left overs. What do you do with the other 2? Well sometimes, even I need to test more than once. (Decorative stitches can be a real bear.) Usually I need only the 3rd piece or if my sample is two-handprints large, I can use only the first 2 pieces. I need the 3rd, to test my fusible interfacing, buttonholes or special techniques like beading by machine. Yep, I nearly always interface my hems. My only exceptions would be narrow turn-up twice hems (Hate ’em. They never stay down.) or serger rolled hems. You might think of something else. You might even not want interfacing in your hems. I do. I like how the extra weight of an interfaced hem helps the garment hang. Some fabrics need a little more weight to hang nicely. You might need more than 4 testing pieces. For instances , if you wanted to do special seams. Myself I like to test both with and against the grain for a rolled-hem edge. I’d rather discover the need to change from rolled-hem to flat-turn-stitch hem at the testing stage rather than ruined-garment stage.
Point is, take the time to TEST FIRST all the stitches, all the techniques, everything that your final garment will need TEST FIRST.
When I was in school I thought that we practiced the first few days and after that we were supposed to be able to sit down and sew without ever testing or practicing again– for life. I’ve found that my sample time is the most valuable, most worthwhile activity for any of my sewing. Not only does it tell you if you have the right stitch and settings correct, but it can help you decide if you really want to do what you thought you did. Maybe I’m thinking rolled hem edges. But I can’t get the rolled hem to clean finish around curves. I’d rather decide at testing phase to switch to a narrow flat serged edge, turn up and stitch, then to ruin my garment. Never resent testing. Never avoid it. Never think it means you are less experienced or less knowledgeable or less anything because you tested first. Never think that it costs time. It will save you time in the long run.