What's so important behind the "Sew first, then Dye" rule?

Well... it can spare you a lot of trouble. Plus, it will spare you remnant pieces in colors you perhaps don't like any more afterwards.

Let me explain the process:

If you buy white fabric for whatever project which you anyway intend to dye, then you have to wash it first at the highest temperature available to wash out any treatments that could influence the dye process in a negative way. This will also preshrink the fabric - the chance that it will shrink considerably more when you dye it afterwards is relatively small.

So - if you have preshrinked fabric then you can start sewing with it right away. Some advantages:

• you can use water removable markers on the fabric for marking your pattern pieces. For example, if you would want to make a black garment and would dye the fabric before cutting the pattern pieces, you would afterwards have to mark these pattern shapes on the fabric with chalk, because nothing else will be visible on the black background. But white chalk is gone quicker than you can think about it. The dye process, however, will wash the marker out.
  
• You can use your remnants for another project which will have different colors. For example, if you have just half a yard of silk satin left from an underskirt, this will afterwards make a nice lining for a bodice.
  But lining should under all circumstances be white or better natural colored, because every dye is somehow chemical - would you like chemicals near or even tightly pressed to your skin?
  
• This process will spare you some dye. If you dye the fabric first, you also have to automatically dye the remnants of your fabric. If the fabric is already cut and sewn, you have less fabric to dye.
  
• The dye process - during which the fabric is washed again - will bring up seams that are not properly finished. So after the dye you can do repairs which you would have to do anyway - but this time you can clearly see where the problem is.
  
• If you have sewn all parts and dye them all together, you can be absolutely certain that you didn't dye less fabric than you will need in the end.
  
• Anything that needs lining, for example a bodice outside, can be dyed *before* the lining is applied (which, as stated above, should be made of a natural colored fabric). It's just so that the cutting edges should be properly finished with zigzagging or a serger.
  

Conclusion: I personally find it much easier to sew fabrics before I actually dye them. There are some exceptions:

• Silk taffeta and silk Duchesse should neither be dyed nor machine washed.
  First, they won't take the color too well (because the silk threads in the weave are too stressed).
  Second, they will get folds in the machine, and in those places the stressed fibers will break, leaving something that looks like folds with dust on them - just that this can't be ironed out any more.
  However, as fabric also shrinks while dry-cleaning (just much slower!), I prefer to water those materials for one or two days.
  For this I lay the materials out flat and with as few folds as possible in my bathtub and drench them with first cold, then lukewarm, then hot water. I change the water several times during this process. The fabric should dry hanging.
  After doing this you can be sure that the fabric won't shrink any more during dry cleaning (which is especially important if you for example make a tight fitting corset of silk duchesse!).
  
• Some other materials, like silk / silk/rayon brocade and Dupioni silk can be washed, but will look different after washing. Not necessarily ugly - but different. Some Dupioni silks develop some kind of "Herringbone weave" during the washing, which I consider to be quite pretty. The brocades have to be ironed while being stressed after washing while still being damp. They will become much softer.
  
• Some materials can't be washed nor watered at all - such as some acetate taffetas. It's best to try it with a small scrap of fabric that is test washed with an ordinary washing load of clothing. If it gets destroyed - try washing another scrap, this time with the wool program of your washing machine.
  
• *Some* materials are said to be not washable, even if they are. For example silk velvet.
  Don't believe the sellers! As long as you wash it *before* sewing, it can also be washed *after* sewing.
  Silk velvet, however, gets even better with a treatment that most fabric sellers run away screaming from. Simply wash it (with a suitable silk/rayon detergent - I prefer Tenestar) and then tumble it dry. I repeat, tumble it dry until it *is* dry.
  If you don't have a tumbler you can also hang it on a clothesline *in the rain* and let it hang there until the rain has stopped and the wind has dried it - and, yes, it can hang there for several days...
  I'm not kidding you - this method works, and I guarantee that you have never seen such lush and shiny silk velvet ever before in your life as after a rain and wind treatment (unless of course you live in the middle of a really, really dirty town right next to a factory, of course...).
  I've heard that this doesn't only work for silk velvet - a friend of mine tried my 'weather' method on a piece of cotton velvet that, according to her, "no one else found salvageable" (I guess she did that because she thought that she might have nothing to loose ) and received amazing results.
  If you don't believe me - try it with just a scrap of velvet (it should be large enough to move in the wind, though - meaning at least 5x5 inch; better a yard x a yard) and report the result back to me - I will happily publish it here.

By the way, I also recommend reading my 'Fabric dyeing' tutorial for further elaboration on silk washing methods.