I usually prewash my fabric, in cluding muslin, but when I do string blocks I use whatever I have on hand. It could be an old sheet...old muslin, you just never know. If its unclaimed for another project, stained or just something someone gave me to give away, I just 'string' it up! Batting is entirely a personal choice.You can even use fleece if you like. Depends on how heavy you want and your budget. Try googling string blocks and then click on 'images' (it will be along the top ..mine says 'You+ Search Images ...." You get more pictures than you ever thought were out there.