If you want your neckline to be snug but also want to get into your suit through it, you'll need a way to make it temporarily larger.
There are a few ways to go about temporarily lengthening the neckline that jump to mind - keyholes (a button closure at the neckline with an open hole over some area of the back) and halter (a band around the back of the neck holding the front up, exposing at least some of the shoulders and shoulderblades at the back) - but those are too feminine for me: I'll stick with a zip.
You can introduce a zip into any seam that reaches the neck. The obvious candidates are the centre front and centre back seams, but if you choose raglan sleeves with the curved shoulder darts, you can put zips into those.
It's pretty natural to put a mock turtleneck collar onto a catsuit with a snug neckline, so I'll cover how a zip works with that.
When you sew a zip to a stretchy edge, the edge should be stretched. Thus a 10" zip might cover 9" of fabric, so when the garment is worn and the fabric stretches to 10", it matches the zip. It's difficult to predict the actual stretch your body will cause, and it's better to slightly under-estimate (so the zip is taut) than to under-estimate (so the zip ripples/puckers).
An exception: if putting zips into the raglan sleeve shoulder darts, the darts are the compensation for the shoulder stretch, so don't put in additional stretch or the zip will ripple.
Two kinds of zip
The alterations to the pattern required by a zip depend on the pattern you've chosen and where you want the zip.
- Most of the suits on this site have a central back seam and sometimes a centre front seam. For these, I would recommend installing a concealed zip into the seam. A concealed (invisible) zip is sewn to the cloth just at the base of where its teeth mesh, so it tends to hold the edges of cloth together in such a way that the zip looks like a normal seam. (It actually attaches to the seam allowance you'd have used on a real seam, so you don't need to make much of a change to a pattern to add a concealed zip.)
- Only the gusset leg body pattern has no front or back seams. If you want a zip for this style, it's best to install a conventional zip because the fabric that's folded out of the way to accommodate the zip's width can be used to attach the zip, avoiding the need to construct a separate panel. A normal zip is attached at the back of the teeth, away from where the teeth mesh, and spreads what could have been a narrow seam apart.
- If you've elected to use a raglan sleeve with shoulder-darts, then you can put zips into those darts, but your collar will have two breaks. Invisible zips would be best, I think.
- If you're making a zipped bodysuit as a base for a mascot-style costume, you should use the sort of coarse/chunky visible zip used with hiking weatherproof jackets, because trapped fur will make a fine zip unusable.
In practice, and despite what many other people say, I've generally found invisible zips much easier to install than conventional zips. They're certainly much less obtrusive, although I've never quite managed to get one to look exactly like a seam. The only problems I've had - they can be stiff to zip past a serged seam, sometimes when sewing them in the needle can bend and sew across the zip teeth, preventing the zip from working there until you snip the stitch. Oh, and you probably want to put a hand-stitch through the end of the invisible zip's ribbon when you're doing the backstitching at the start/end of the zip (the nine backward stitches so you don't have to knot the thread) so you can still pull it when most of it's under the zip foot, instead of trying to pull on the stretchy lycra and potentially sewing in wrinkles.
I'll cover this before I cover the zips, so the zips' sewing sections don't have to mention what the zip does at the collar. There are many types of collar, this is one of the simplest.
Very easy - a rectangle of cloth, as long as the neckline you'll be attaching it to, and twice as wide as the collar should be (plus seam allowance, naturally).
- With the neckline assembled except for the seam(s) you'll be putting the zips into, pin the collar to the neckline, right sides together, and sew.
- Lay garment right side up.
- If you're installing a visible zip, secure its base to the little tab at the bottom of your inverted Y cut. (See visible zips below.)
- Lay the zip along the seam (including half the collar), with its teeth pointing away from the edge and the zip facing down. Sew along the zip's teeth with the zip foot.
- Lay the other half of the zip along the other side (including half the collar) in the same way, and sew along the zip's teeth with the zip foot.
- Fold the collar over, so its free long edge overhangs its other long edge's seam by the seam allowance, and sew the short edge of the collar over the zip's ribbon, making sure that the extra ribbon at the top of the zip is pulled away from the side with the teeth. Remove most of the extra ribbon.
- Turn the collar right side out, then sew its free long edge to the long seam joining it to the neckline.
Invisible zips' ribbon joins the zip just behind the teeth (and curves a little past the join site) so the zip pulls the garment fabric (blue) right up to the teeth on either side of the 'seam' and is (almost) invisible when fastened. In the diagram, the teeth are coloured green.
The pattern changes required for an invisible zip are minimal: just mark the zip along the seam you're going to hide it in, bearing in mind that you will want to reduce the zip's effective length on the pattern so you can stretch the fabric to the right degree while sewing it in, so that when worn, it will lay flat on the body. (But typically, no stretching for shoulder-darts.)
This requires a conventional sewing machine and a zip foot. If you have an invisible zip foot too (with two grooves running down the middle) then so much the better.
- Close the seam to an inch or two below where you've marked the zip's end.
- Open the zip. You should cool iron the zip so the zip ribbon is flat and the teeth stand up; once ironed, don't close the zip until it's sewn. (I tend just to use the zip foot to hold the teeth vertical, which is probably why my installations are visible.)
- Lay the garment right side up. Place the zip upside-down (that is, teeth uppermost since this is an invisible zip) to one side of the seam, with the teeth running along the seam line and the free edge of the ribbon along the cut edge of the seam allowance. (Diagram A)
- Pin perpendicular to the zip at the top tooth and just above the zip key (which should be at the bottom), then stretch the fabric until the zip is flat. Pin perpendicular to the edge, half way between top and bottom; then again into quarters, and into eighths if you think you need them. Release the fabric - the puckers should look even.
- Lay the other side of the garment out, and turn the unpinned half of the zip over it in the same way, so that when you turn the zip inside the garment, the joined end doesn't twist nastily. Stretch and pin.
- Once you're happy that the pinning is even, that it matches on both sides, and that the joined end of the zip will wind up in the right place in the garment once sewn (turn the garment to check), sew the zip to the garment with the needle close to the teeth (using the invisible zip foot's central ridge to guide the zip through the machine). (Diagram B, except that you will have sewn much closer to the teeth, and will have sewn both sides of the zip. Diagram C is diagram B with the zip and seam allowance to which it's sewn, folded under to show the teeth half-visible when viewed from the top.)
- Having sewn both sides of the zip, you need to continue the seam below
the zip (step 1) to just above the end of the stitching that attaches the
zip: pull the zip closed and lay the garment with right sides together, seam
allowances pulled around the zip. Sew (red in diagram D) with a normal zip
foot, from just above the zip's lower end (the vertical dashed line), down to
where you closed the seam in the first step (the hatched area at the bottom
(By 'normal zip foot', I mean a foot which only covers half the plate: many invisible zip foots incorporate visible zip feet along their outside edges.)
Visible (normal) zips' ribbon joins the teeth at the back, so the zip isn't hidden at all. Visible zips are much less likely to get caught on the garment's fabric.
I only recommend a visible zip when you don't have a seam to install a concealed zip into.
- Mark a vertical slit down the pattern, for the scaled length of the zip (so the zip will lay flat when the garment is being worn) from the stop at its top teeth to the stop at its bottom, minus half the height of the collar, and minus the height of the stop.
- Mark a right-angled triangle at the bottom of the slit, so the vertical slit looks like an upside-down Y. The triangle should be half as wide as the zip+ribbon.
In the diagram, the pink is the inside of the garment; the zip will be sewn to the zip along the folded edges, and will face away from the inside (ie the zip key will end up on the outside, as you would expect).
- Cut the slit. Around the bottom of the slit, sew a reinforcing rectangle just under half as wide as the zip's ribbon. (This is just sewing through the garment fabric for strength, not sewing anything to it.)
- Cut the right-angled corner, the tab, from the base of the slit to the corners of the rectangle.
- Lay the garment right side up, with the zip both upside down and back to front, so its top points towards the bottom of the garment, and its bottom is across the tab; the key should face downwards. Stitch the loose ribbon below the zip's bottom stop, to the widest end of the tab.
- Turn the garment inside out and flip the zip so its top points towards the top of the garment and it lays atop the garment in its final installed position (the tab will be folded under the zip, pointing downwards, right side towards you, green in diagram).
- Open out the slit, wrong sides together (to reveal the right side, yellow in diagram), and align the cut edge of the slit with the edge of the zip ribbon. Pin at top and bottom, and half, quarter, and eighths along the zip (unless you're confident enough to pin only top and bottom and stitch it under tension). Remember to stretch the fabric as much as it will be stretched when you're wearing it - the zip should just come taut at that point.
- Fold the garment along the zip (zip outermost) the other way and align and pin the other side of the slit along the other side of the zip, at top, bottom, half, quarter, eighth etc. Check that the puckers between the pins are even, and that the two sides match.
- Stitch along the ribbon, halfway between ribbon edge and teeth, following the collar instructions, holding the fabric taut if you took garment stretch into account.