So, physics demands that special steps are taken in order to successfully press a curved seam.
So, there are typically two results of a seam. Either the two pieces of fabric sewn together will be open and lie “flat” or as flat as the seam will allow it. Or, the two pieces will be folded back on itself wrong-sides together sandwiching the seam allowance in there. This is the case for collars, facings, bag flaps, and waistbands.
It doesn’t matter. The physics remain the same.
In a concave seam, the seam line (the line you sew on) is longer than the seam allowance or edge of the fabric.
This means that if you try to open up that seam, the shorter seam allowance will have to stretch.
Even in a knit fabric this could be asking too much. And will likely distort the shape of the seam you are pressing.
In general, you just need to make tiny snips in the seam allowance from the edge of the fabric almost to the line of stitching. Be careful not to clip the stitching.
These little slits will allow the fabric in the seam allowance to open up and let the seam lay “correct”. I mean it’s curved, so it won’t be flat.
You can also choose to trim the seam allowance to about 1/8″ – 1/4″ if you didn’t already sew it at that.
In a convex seam, the length of the edge of the fabric is longer than the seam. And this means that when you try to open out the fabric the seam allowance will bunch up and ripple.
The way to get rid of that is to make small V shaped notches in it.
This removes the bulk in the seam allowance thereby allowing the seam allowance to lie flat against the main body of the piece.
And, you can also trim the seam allowance to 1/8″ – 1/4″.
For either type of curve, make more clips in tighter curves.
Where possible, I recommend pressing the seams open first before folding the fabric on itself. If it is this is where that pressing ham comes in handy.
It’s not always possible.
For something like a pocket flap, you may not have the ability to really press it open.
In this case, roll the seam allowance in your fingers while sandwiched between the main pieces to work it out so that the seam is in the middle of the two folds of fabric.
Get it as good as you can before putting the heat on it. However, sometimes it’s a little easier to work with the fabric while it’s warm and/or steamed. But be careful not to burn yourself.
This is much easier demonstrated.