On fabric,
muslin fabric comes in bleached and not bleached. Not bleached will absorb water,
starch, paint and glue slightly better because it’s not already saturated. Bleached fabric in addition to being more expensive, will also due to the bleaching process be less strong or available in wider widths. The fabric is also available in differing weights of material which would be it’s overall
thickness. This weight will have an
effect on how smooth the fabric is, also how much tooth or holding
power and stretch it will have. Something that’s as smooth as linen well due to the proximity of threads to each other and thread count, shrinks to conform to a
flat differently than something that is rough, less refined and has larger holes between threads. The heavy weight 128 to 140 thread count fabrics are fabrics with a better tooth to it and larger shrinkage, thus the fabric you want to use because it will also be stronger. It has been a long time since I faced a soft
flat so any further info on the subject is off what other people advise on the subject or is listed in books about it.