Piping is designed to strengthen a seam, as well as add a decorative element. In this vein here is a quickie guide to piping in stretch fabrics, especially PVC where mistakes can ruin an entire garment very quickly due to stitch marks leaving irepairable holes.

Stretch PVC lives up to its name and so can make piping particularly difficult as you are sewing through four layers of fabric all of which are trying to move away from each other and from the foot and the platform of the machine. This method helps prevent that and prevents weakening of the fabric.

Firstly when patterning your pieces decide on how wide to leave your seam allowances. I recommend at least as wide as half the regular foot of your machine. This is because the pressure between foot and feed dogs will make the fabric shift (usually to the left if the fabric isn't perfectly lined up with the feed dogs) which will then cause fabric to tend to bunch up under the foot. If you leave a few mm extra (or say 1/8") then this is much less likely to happen.

Next you need to choose the cording. Naturally with a stretch fabric you want a stretch cording. You can buy it by the metre/yard or prepackaged. I prefer by the metre as I get a wider variety of thicknesses to choose from. I recently found an ideal elastic in gold of all colours...

You then need to cut your piping strips and this will be twice as wide as your seam allowance plus the circumference of your piping. You could go mad with math but generally guesstimating will work. Fabric is not paper and will stretch more around 3D objects.

So next up set your sewing machine to a wide and deep zig zag. about 3/4 of your seam allowance will be fine, or the widest setting if it is a regular machine.

Align the top of your first piece with the top of one strip right sides together. Overlap the strip by about half an inch or 1.5cm. You will want this later on. and zig zag right on the very edge to secure the two fabrics together. You will not need pins for this just a little patience as you feed the fabrics under the foot.
No need for securing the stitches either at start and finish just cut the strip with half an inch or 1.5cm extra.

Once you have sewn one side then match up the other side of the seam with the matching panel:



This is the side back seam of a bodysuit with full length piping.

As this seam is rather curvy it is extra prone to the fabrics stretching unevenly under the foot so a few pins as close to the edge as possible are used to make sure the strip and both side pieces are matched up with no extra stretch anywhere.



Next you repeat the zig zag stitching along the other side:



It is possible to skip the above steps, but it really does make the following infinitely easier and more importantly neater than if you didn't.

Next you need to set your machine to the simplest stretch stitch. This is seen here as stitch "14". Essentially it is two or three forward stitches with one or two back stitches.



You also will need a piping foot.

Now you take your cording and place one end at the top of the strip and match edges of the pieces and place under the foot of the machine.



You will need to to a little tweaking of where the needle sits (as far left as possible without going through the cording) and where the fabric sits under the foot.



Sew along the seam. Again you do not need to secure the ends, the stitch is already set to go back and forth.

E voila, a piped seam:



For the intersection of the short seam and long seam: The short seam is sewn first then the two full length panels sewn (long seam). The long seam is sewn from top to intersection, then bottom to intersection and then very carefully hand sewn at the point to neaten. This is to reduce bulk from the piping and multiple layers of fabric.