There are exactly three (3) things that blocking does well:
- Stretch out a gossamer piece of lace so that it struts its lacy stuff.
- Allow the yarn to settle into its new shape gracefully.
- Smooth out small irregularities.
Blocking works very well for lace knit on fine yarn. Lace stitches like to be open, and blocking can help them do this. In a fine yarn, blocked lace can hold its shape well for a long time.
In most other situations, however, blocking can't do anything that dramatic. It can help shift yarn to neighboring stitches so that a piece looks more beautiful and finished, but it can't drastically alter the shape of a garment.
In laceweight yarns knit on relatively large needles, the knitted item doesn't have enough shape to combat the blocking. You block the piece, and there is not much yarn to pull back against your blocking. If you work with worsted weight yarn, though, the resilience of the yarn will pull back against the blocking and the item will re-assume its natural shape.
With a heavier yarn, therefore, you might be able to block a piece into shape, but it won't hold that shape very long. When you put it on a moving human body, the yarn will shift back around until it gets comfortable. Gravity will take its toll. Quite soon, the knit item will assume the shape it wants to be instead of the shape you blocked it into.
In some cases, such as flipping borders, a piece won't even hold its blocking long enough to dry. As it dries, the character of the knit will reassert itself.
Often, people suggest blocking a piece of knitting to change its size or correct a major problem. I don't know of any situation where this has actually worked. A person can confidently block the devil out of a piece. They wear it once, it assumes its real shape, and it gets stuffed in the back of a drawer forever.
If you have a piece of knitting that is:
- Too big,
- Too small,
- Too wide and too short,
- Too narrow and too long,
- Has a border that flips out or in,
- Has edge curl,
- Has a collar or trim section that won't lie flat,
Then blocking won't fix the real problem. Blocking can't change the character of knitting.
Knitted items have their shape because of the properties of the yarn and the knit stitches used. Knitting is inherently three-dimensional. We use this to advantage when shaping hats and sock heels. It comes into play when we use pattern stitches as well, even when we don't desire the three-dimensional effects.
If you knit a plain piece of stockinette fabric, it naturally curls toward the knit side on the top and bottom edges and towards the purl side on the vertical edges. The natural shape of a flat piece of stockinette fabric is not flat — it's more like a potato chip with rolled edges.
Experienced knitters know that stockinette is not a good stitch pattern for flat items, nor is any stitch pattern based on stockinette. For flat items, you need a balanced stitch pattern. The knit and purl stitches need to be arranged in such a way that they counteract one another's tendencies to curl.
Stockinette is great for items that encircle a body. The vertical tendency to curl inwards helps the item hug the body. Anywhere you have an open edge in stockinette, though, you need to do something to counter the fact that stockinette edges curl. If you do this properly, the item will hold the shape you want it to. If the edge is not strong enough to counter the curl, however, you will have borders that flip or otherwise misbehave.
Other stitch patterns have other tendencies. Ribbing is elastic widthwise (and thus draws in widthwise) and not lengthwise. Garter stitch compresses lengthwise and spreads out widthwise. At one time, these two stitch patterns were often used as complements -- garter stitch on vertical borders and ribbing on horizontal ones.
Moss stitch, seed stitch, and all members of that stitch family have a strong tendency to bias, particularly when they are knit from singles. These stitch patterns are also wildly expansive. They can grow lengthwise and widthwise with wear.
Understanding the way different yarns and stitch patterns work can help you plan garments that work. When you want to use a new yarn or stitch pattern, make a nice big swatch. When you're finished, let it sit for a day. Wash it, smooth it out, and let it dry flat. Play with it. See what directions it stretches in. Notice whether it has biased at all. Thread a knitting needle through the top and bottom edges, hang it up, and hang a little ball of yarn from the bottom edge. Does it stretch much after an hour or two?
If you end up with a piece of knitwear that has a problem, you can try blocking and see if it fixes the problem. In many cases, though, you might not be happy with the results. In that case, you can give the item away or you can rip out the parts that don't work and make something you will be happy with.
The ability of knits to be ripped out and re-knit is one of their big strengths.