So, I decided to leave the wacky world of underwiring for now (no doubt I will return there later) and concentrate on making something (relatively) quick and wearable. I had some nice quality black lycra in my stash (bought for my retro swim suit but there was plenty left) and bought more new elastic. The lycra actually matched my old H&M hipster bikini bottom, so if all else failed, I'd be fine if I just made one OK top.
I planned on making a very simple longline bandeau top (a.k.a. a boob tube) to match the old hipster, and a high-waisted bikini bottom with a gathered bandeau top. The longer top and the bottom were drafted based on my lingerie slopers and for the last piece, I used this tutorial from pattern school. I really recommend that site: lots and lots of information about 'close-fit stretch-wear patterns' i.e. swim, dance and exercise wear. Just no real sewing instructions, so only easy to use if you sort of know what you're doing. My bikini top was drafted completely according to the instructions for the gathered bandeau, just with a smaller hole at center front and with the strap knotted at the top of the bandeau.
I'm very happy to report that this time, it all worked out.
I even had the audacity to try that blind finish again on the high waisted briefs and the gathered top. This time, I found a better lining, calculated the turn of the cloth better and it worked like a charm.
Here you can see the top edges of the longline bandeau, with a conventional finish.
And here are the top edges of the gathered bandeau, smooth on the outside and with the seam and a zigzag on the inside.
You may notice some waving in the elastic (especially in the top picture). Although with the tops flat on the table, this may look like a problem, it is a solution. This stuff, Gold-Zack 1 cm wide transparant elastic, was the only thing remotely like swimwear elastic I could find. It looks the part but I found out in my attempts with the grey bikini just how sturdy this stuff is... So I decided to stretch it slightly less than the lycra to prevent uncomfortable and unsightly cutting.
By now, you may wonder what these bikini's actually look like... Well, here we go:
RTW bottoms, longer top.
High waisted bottoms, gathered top (by the way, this thing is surprisingly supportive)