Quote from Sulky site.

Prepare the Fabric
To remove any finish, wash and dry the poly/cotton fabric. (100% cotton will not hold the crayon and will fade with washing.)

Nope...I never prewash & I always use good quality 100% cotton . I do make sure I heat set it, and I have never had any problems. A couple have been washed several times and are still as bright and clear as the day I made them.