I'm in Australia so probably not a lot of help, I think your muslin is similar to our calico or homespun and no, it very definitely shouldn't be see through, don't put that anywhere near your quilt. Even in our calico there are different grades of quality, the type you would want to use for quilting should be no less substantial than your cotton quilting material. I suppose when it comes to internet buying the term 'buyer beware' is a good one to remember.