#4 Finished Closed Bottom (10",20",26")
I stock all of my standard 94 colors in #4 or MEDIUM weight invisible zippers.
Invisible zippers come in 3 weights: #2, #4 & #6
#2 are the standard that you find in fabric stores. (2mm across the coil)
#4 or 4mm across the closed coil, are a little heavier & stronger. Use in bride's maid dresses, pillows.
#6 or 6mm across the coil, are the size that Vera Wang uses in her gowns & I use in cushions or slipcovers.
To make your selection of the correct size invisible for your project a little harder, some manufacturers call these #3 & #5 invisible, instead of #4 & #6. My coil is a bit heavier but will normally fit #3 and #5 invisible pulls.
The little plastic invisible foot that you have will NOT work with the #4. Being cheap, I just use my standard zipper foot & stitch in the ditch. If you have a commercial machine, I stock Sam Sloan's adjustable foot in "Thread & Workroom Supplies".
This finished or closed bottom zippers work well in dresses or if you only need a couple zippers. They are easily shortened from the top or bottom. If I'm applying an invisible zipper to a dress or gown, I don't attach the bottom of the zipper to the seam allowance, but let it hand free (keeps that little stupid pucker at the bottom of the zipper from happening). Videos above to show you how I just bar tack & cut off the zipper at the bottom.
If you need longer lengths, try my yardage. You may purchase "Pull ON the tape" & I'll put the pulls on for you. Just slide them down to the length zipper you need & clip off.