did you originally wash the fabric with some kind of color catcher? i use retayne. but others here have used household color catchers or color grabbers. even after quilting, i have used retayne where i thought/knew there would be a problem. i just wouldn't wash it without first sandwiching. can you baste thoroughly to a sheet, with either pins or thread, and then zig-zag the edges? the back side needs to be well-protected.

washing gently in cool or tepid water was good advice, too.

I would absolutely not wash it in the machine. It's not just the edges that can ravel; every seam can lose threads and create a tangled mess.

What I would do is machine baste it to a muslin foundation and then hand wash.

Edit: Scrolled down and read your second post. You could hand wash as described above in Synthrapol, being sure to use lots of water in the sink and replacing the water frequently if you can see dye in the water. Synthrapol should keep the dye from settling into other fabrics.

What kind of batting do you have? Polyester batting will not shrink. If it's cotton, cotton/poly, or wool the package will tell you if you can prewash. Poly doesn't need prewashing, of course. Cotton batting that is bonded and/or needlepunched can be soaked (as in a washing machine with *no* agitation), spun out, and then dried in a dryer to shrink. Traditional cotton batting cannot be prewashed; it will fall apart in the water.

Lay the whole top out flat to dry, then take off the basted foundation and block to size.

if this is strictly for a wallhanging, i wouldn't wash the batting at all. i would wash the quilt top as described by the two methods above and never wash it again.
shake the dust loose, vacuum, air it out in the dryer or dry clean, but if you ever wash it again, it will crinkle to some degree unless the batting is 100% poly.

prism, what do you think?

Actually, this doesn't take blocking out of the scenario - it makes it ideal.

I routinely wash my quilts in the washer, put them in the dryer for less than 5 minutes, then block them. Wet fabric is easily manipulated into position, which makes it way easier to block. If you're concerned about bleeding make sure you use a color catcher in the washer, and take the quilt out right away - don't let it sit there. Once you've blocked it and pinned it into position, use a fan to help the quilt dry quicker.

Your quilt will be flat and square. FYI: this is how many quilters prepare their quilts for quilt shows. It really does help make the quilt hang better, plus it helps the batting poof up a bit without crinkling.

I always wash and dry all quilts and comforters as soon as the binding is sewn on and of course quilted.

Don't prewash your batting, just put it in the dryer for about 15-30 minutes. And use poly batting---less scrunchy look after washing.

On 2 quilts where I used Bali pre-cut strips, I did not wash the fabric first, knowing that some of the batik colors would run. When I finished the tops, I machine basted wide, inexpensive muslin all around the quilt top edges and then washed (gently) with color catchers. After laundering, I removed the muslin; the seam allowances were intact and the muslin prevented unwanted threads from escaping into my washing machine.

Yes, it will. You can prewash batting, which I am taking to mean cotton batting, by handling it very gently and drying it very gently.
Works fine.
Do not wash it in a washer, nor dry it in a dryer.
Although a friend of mine hand-soaked her cotton batting, rinsed it by hand, and then put it in a pillowcase and pinned it closed, then dried the whole thing on warn (not hot) setting.
Hers came out fine but I have not done the dryer thing myself.

Okay, this sounds like the plan of action I need to take. But the only muslin I have is some nice, really soft stuff that I plan on using for a backing at some point for a cuddle quilt so I guess I need to hit the store for some cheaper stuff. Couple of questions; you just basted around all four edges right? Did you just wash on a gentle cycle? And finally what about drying? Did you dry in a dryer, or just hang to dry?

There are inexpensive muslins available.

Or you could buy the cheapest (least expensive) sheet you can find and that would work.

Even hand washing just a top is an iffy process. I had to wash some sections of a top (one of my cats "anointed" it - thought I had everything away from the wretch) - and there was a LOT of fraying. This was just dunking the sections in a sink and hanging them on a drying rack. Also, some of my stitching came out and I had to redo some of it.

Warm and Natural batting machine (automatic) washes (warm or cool wash and gentle cycle) and machine dries (permanent press cycle) just fine.