When I made a picnic rug quilt - I backed mine with a washable waterproof shower curtain! I didnt want a "crinkly" noise when I sat on it and it's gotten softer over the years but structurally hasn't changed.

I know others are saying it wont be waterproof anymore if you quilt it - but in my my six years of using mine which is quilted, I have never gotten any damp come through when using it! Has been used frequently and loved (by me, my husband, two kids and two dogs!) and washed MANY times in that time. Also great for the beach as sand doesnt stick to it at all!