With thanks to some of the experienced members who had seen 2 thread overcasting done with an overlocker, this is what I had in mind with the technique. An open seam is an uncontentious method of seam construction in a wide variety of garments but it faces a common problem across any garments that need to be fed through a washing machine, securing the folded back edges so they don't end up in your washing machine's lint filter as the garment falls apart.
As I have bothered to set up one of my overlockers to do 2 thread overcasting, this is what I have done the test piece with but I should note that if you have a 3 thread overlocker, especially if its a lighter construction type you can use a very fine thread like Coats Gramax for the two looper threads, you should be able to achieve similar results if you iron it flat as I have done with the 2 thread overcasting.
Secured the two edges with overcasting or if you only have a 3 thread machine, use a narrow cutting width then iron it to flatten the overcasting or overlocking as much as possible. It does not matter that much if you slightly fry the overlocking thread, the idea is to get the edges as flat as possible.
Pin your garment together and mark out where you are going to sew the seam that joins the two layers.
Try and be a good shot when you stitch along the chalk line. In this case I have used a reasonably small stitch length which holds the seam together more accurately when it is flattened out in the next operation. but note that this comes at the price of making it harder to unpick at a later stage if the garment is going to be modified.
Lay the seam out and from the back pull the two edges apart so that it ends up flat. Then you iron it to get it as flat as possible.
If you have pulled it apart carefully and ironed it, it should look something like this.
This is the finished appearance of the ironed front of the seam. I tested ironing over the front using an ironing cloth and the fabric in the folded over area slightly shows through but importantly the overcast edges do not and that was that main idea. Te solution if you are ironing the garment yourself is to iron the seam from the back lifting the two edges so that you do not iron over the folded back sections. If you are making a professional quality garment, Claire's advice is the right one, send it out for pressing to a professional who knows how to press quality garments and make sure you get the results you are after.
If there is one thing to stress when you aspire to make better quality garments is the standard of your workmanship. I get to see a fair share of very good quality jackets and a factor they all have in common is a very high standard of workmanship and in this case, the close detain of the seams used in the jacket's construction. This example is done out of some scrap drill I use for prototyping but you should be able to get the general idea. Attention to detail puts your work well above the junk that is shovelled out of Indo-China if you make the effort.