I would not prewash. I used to prewash everything but I haven't done that in years. You can throw a couple of colorcatcher sheets in with the quilt when you wash it and they will absorb the extra color while it's washing. Last year, I did a quilt for my grandson using gradated fabrics that ranged from a very light beige to an almost black dark brown. I threw in the color catchers and there was no bleeding of the dark onto the light.

First, you don't say what size the precuts are, so I'm assuming charm packs. I'm thinking that would be a bear to sew after washing. You will have to iron each square, etc. Then, they probably aren't going to be square anymore, plus they would have raveled to some degree. It doesn't sound like sewing them would be a good experience.

All and all, I don't think I'd want to deal with what I end up with after washing. I might wash a couple of squares to see, if you are convinced it will improve the outcome. Then, you will have a better idea of how washing will turn out.

Since it's precuts, it's all made by the same manufacturer. So, I would assume the squares will all shrink the same amount.

I agree, that's a nice assortment.

bkay

This is what I would do, too. I would leave the squares in the lingerie bag while drying in the dryer, though. I have also safety pinned pre-cuts together before washing and then put the pre-cuts into the lingerie bag to prevent the raveling.

It's a pain, but it works.

I would use what you have as is. This way you can see for yourself and you may be pleased or you may learn a lot.

I think prewashing is an invitation to disaster and making the kit unusable because of the raveling. Even if you wash and dry in a lingerie bag it will still ravel a lot and then it might shrink unevenly making it unusable whereas once quilting the batting will control the shrinkage. I would just use color catchers in the wash but I rarely prewash including flannel but I only buy flannel from connecting threads and it has minimal shrinkage.

I agree with those who recommend not prewashing. In terms of shrinkage, fabric (including flannel) shrinks very differently after being quilted than before. What I would recommend is quilting a large X in each block. This will stabilize the layers so they shrink together.

As for colors bleeding, I like to do the first wash with Synthrapol in a machine that uses a lot of water. Most domestic front-loaders do not use enough water to dilute dye bleeds. Domestic top-loaders and large front loaders at laundromats use sufficient water. Very few fabrics bleed these days, but it's always good to have insurance. Synthrapol helps keep loose dye particles suspended in water so they get rinsed away instead of settling into fabric. Although it's most effective with hot water, for this kind of flannel quilt I would use cold water and add a few color catchers along with the Synthrapol. That should be more than adequate, especially if you use lots of water for that first wash.

One thing to watch out for with flannel is pilling. To minimize pilling, always wash the quilt by itself and preferably in a front-loader. Top loaders that have a central agitator posts are hard on fabric because of the rubbing of the quilt against itself. Avoiding that kind of friction will help minimize pilling.

PLEASE don't prewash, please DON'T prewash, please don't PREWASH.

What jokir44 said!! In spades!!!

I am a pre-wetter. I wet down most everything that comes into my house because I have an alergy problem with the sizing in most fabric. I rinse stuff in as hot a water as I can stand and then dry in a dryer. It gets rid of enough of the sizing so that I can work with it comfortably and also controls shrinkage. But precut flannel is something that I would never even think about pre-wetting.

Why not fill the washer with hot water and turn it off to let the item soak, not agitate, then select drain and spin. Much easier.

Onebyone, I have a front loader. Plus, by using a sink full of water, I can weed out and selectively treat any bleeders.