Any knitting project that has pieces that need to be sewn together can benefit from blocking, but it's also great for making projects square and making things come together better for sewing, fit better or even just look better.
What is Blocking?
Blocking is a method of stretching and shaping a finished knitted piece to reach the dimensions suggested in the pattern, to make two pieces that need to match the same size, or to make your stitches look nicer and more even. Lace almost always needs to be blocked to "open up" the design so all those beautiful holes and patterns show in their true glory.
Before Blocking After Blocking
There are three main methods of blocking: wet blocking, steam blocking and spray blocking.
Wet BlockingWet blocking knitting is an appropriate method for blocking man-made fibers, novelty yarns (except for some very delicate novelties that may fall apart when wet—check the label for care instructions and don't wet anything that says it shouldn't be wet) and some wools and wool blends if done carefully (remember heat and agitation can cause wool to felt).
Dampen the knitted piece so that it is wet but not dripping. Spread the piece out on a towel, sheet or clean garbage bag (the bag won't absorb water, allowing the piece to dry faster) on the floor or a spare bed where it can sit undisturbed long enough to dry.
Gently stretch the piece as needed to meet your blocking goals. If you're trying to get a piece to the size or shape the pattern recommended, you might need a ruler, tape measure or yard stick to help you out.
Use rust-proof safety pins, straight pins or T-pins to hold the knitted piece to the towel or sheet. As the piece dries, it will retain the shape that you gave it.
Different knitters use different techniques when it comes to steam blocking. Some people stretch and pin their work to the desired shape before steaming, using the steam to help set the new shape. Others steam first and then pin, allowing the steam to relax the fibers and make it more pliable.
The method you use may depend largely on the flexibility of the knitting. If you can get it into shape without the steam, pin first. If not, steam and then pin.
The steaming method involves slightly dampening a clean sheet or other piece of fabric and placing it over the knitting. Use a hot iron to press very lightly on the sheet. Don't press like you are actually ironing, you're just pushing the steam through the sheet and into the knitting. Continue this process until the sheet is dry.
You can also steam block without a protective layer of fabric. Just set your iron on steam and wave the iron slowly over the knitting, being careful not to touch the work with the iron. Then pin if necessary and leave to dry.
Spray BlockingSpray blocking is the most gentle blocking process and is great for expensive and delicate fibers like silk and cashmere. It's a good method to use when you aren't sure what kind of yarn you're dealing with.
All you need to do is pin the piece to the desired dimensions and lightly spritz the finished piece with water from a spay bottle. Get it damp enough to relax the fibers, but not soaking wet. Allow to dry and you're done.
There are tools especially made for blocking knitting, which you may or may not find useful. Blocking boards are made of heat-resistant materials and often have grids printed on them so you can easily measure your pieces. They can be pinned into and usually fold for storage.
T-pins are often recommended for use when blocking knitting. They are like regular straight pins except the head is shaped like a T. They a long and easy to work with, and also rust-proof so you don't have to worry about leaving them in your knitting while it dries.
For blocking lace and other large projects, you can purchase blocking wires, which are flexible metal wires that can help you block curves or the sides of a large project. The wires can be woven in and out of the project and reinforced with straight pins if necessary.
When i first started knitting I ignored the instruction "block garment pieces before seaming" because I didn't see the value or the point of it. It wasn't until I had been knitting a while and wanted to even out my stockinette stitch that I learned how to block. That and, no matter what I did, even if the pieces measured so that the armhole fit, it always was uncomfortable, the fabric pulling strangely because I have a bust. Blocking makes your life so much easier. You shape the pieces into the way you want them to live. It makes your knitting more even and sets the stitches. And you can fix a lot of boo-boos that way.