I use a layer of Quilters Dream Request white cotton batting on top of wool (QD wool is wonderful) or natural (unbleached -- I use Warm and Natural) cotton batting for white quilts. It's the thinnest white batting I could find, and I found a layer of batting to be a bit easier to work with than a layer of fabric. And it does a good job of eliminating the seam shadowing that sometimes appears when the batting isn't as white as the fabric.
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Handmade quilts in all styles and sizes - from my studio in the beautiful Smoky Mountains of NC
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