Hiya!
TL;DR
Why and when is interfacing needed in the welts?
Could it be a possibility to not sew the sides of the welts down at the early stage, but to do this when I am stab- and *****-stitching the welts, just before I insert the lining?
For whatever reason, I am currently struggling immensely with my welts.
Lacking linen holland, I tried to use the same light-weight canvassing for my pockets, as a stabilizer on the left side and for the welt itself.
=> BAD IDEA! It has way too much "spring" in it. Even if you press it down like a berserker - your welt will be way too thick and sturdy.
(As there was another small issue with the dart in one of the foreparts, this attempt is nw being "cannibalised" for the next attempt.
So, I cut new foreparts and chose to use silesia for stabilisation and the welt. (@Rory Duffy - Master Tailor does so in his coat series, if I recall correctly)
The top pockets* worked quite okayish.
Now, the main issue still is: Sewing / attaching the welts to the rights side of the foreparts.
I've got a rather springy, tightly woven cloth here, 180g - apparently with small percentage of Lycra in it (Which was NOT on the label 😡).
Together with the silesia in the welt, it is a royal pain to get these welts pinned down and sewn correctly.
Therefore, I wonder:
Why and when is this interfacing needed in the welts?
Could it be a possibility to not sew the sides of the welts down at this stage, but to do this when I am stab- and *****-stitching the welts, just before I insert the lining?
* There is a reason, why Rory starts with the bottom pockets. So far, I have managed to sew through the top pockets 3 times (Yes, three times!), whilst battling with my welts! The reason why I started with the top pockets being the parts for those already cut for the precious attempt. I was too lazy too cut the bottom pockets at that moment.
=> NOTES:
Even on a "restart": Make sure all your parts and pieces are cut
Start with the bottom pockets
With respect to interfacing welts, what you need (if anything) depends on the weave and thickness of the material. Some material (drill cotton, for example) is so stiff it doesn't need much, if anything, in the way of reinforcement.
Any number of cheap fabrics can be used for interfacing. All you're trying to do is create some stiffness so that the pocket doesn't sag and is durable. Historically, linen was used because it was so cheap (ironically, not necessarily the case these days).
You can try: a leftover scrap of something; thick pocketing; the material itself (if you have enough to spare). You can get an idea of the interfacing's suitability simply by sandwiching some between a fold of fabric and flexing it. No sewing required.
If worst comes to worst, fusing.