The exception that proves the pressing rule

Mary left a comment on my last post asking why I don't press the seam (attaching the garment piece to facing or lining) before understitching.  Isn't this inconsistent with the usual rule that you must press every seam before going on to the next step?

Great question!  Like Ann (Gorgeous Things aka The Pressinatrix) I am a firm believer in pressing.  During construction.  Pressing while you go makes a HUGE difference to the look of the finished project.  Not pressing will 100% guarantee that your finished garment will look home made in the worst way.  This is why I've invested in a great iron, a ridiculously expensive ironing board, and a lot of pressing equipment.

So yes, Mary, always press a seam before doing anything else to that seam.

Except ... experience tells me that you get a better result if you do not press before understitching, and that in this specific situation, pressing is a waste of time.  They say an exception proves the rule...

Why?  Well, bear with me as I return to the goal of understitching, which is to exactly preserve the shape of the sewn seam by allowing you to easily fold the two pieces along their curved edge.  The finished edge will ideally be perfectly curved with no rippling, unevenness or other distortion, and it will lie perfectly flat with no bulk.

The problem is that the cut edge inside a sewn oval (such as the neck or arm opening) is always smaller than the seam line, as illustrated.  This means the seam allowances have to be stretched (distorted) to be folded back, and will always want to unfold themselves.  They will take the thinner (lining) or shorter (facing) edge with them.

Pressing this edge without understitching just doesn't produce as good a result.  Understitching is more aggressive with the pesky seam allowances than pressing.  It sews them down flat in their stretched position, rather than just folding them back.  Sewing the seam allowances to the lining or facing also firms up the curved edge.

OK, so why not press first and then understitch?  Well, because you just don't need to.  It's an inferior way to achieve your ultimate goal.  And pressing first does not help you understitch.  In fact, it tends to make the job a little bit more complicated.  It's hard to press this type of opening without building in some distortion.  This can be from stretching the fabric as you try to open the curved seam exactly, but more typically in my experience you will find that you have pressed in a fold which is not precisely along the stitching line.  If you are going to understitch (always!) you end up having to correct this fold as you understitch.

So pressing (but ONLY in this one situation) is (1) unnecessary as well as (2) difficult and (3) counterproductive.

Clear as mud?