Sounds like drapery lining. I say that because of the reference to 'rain no stain'. However, it was made in several weights or qualities. Roc-Lon is also the producer for premium quality muslin. I use it a lot for the backing or lining of all kinds of projects. But, as to the fabric you have, I would cut off a piece, maybe even a yard. Measure it very carefully and wash it, dry it, iron it and remeasure it. You can then act accordingly. If it shrinks a lot but the rest of the characteristics are OK, just wash it all-maybe cut in increments so it is easier to handle.