I am a muralist and I am planning on buying the flame retardant pretreated muslin, mainly because I hear it shrinks less when stretched due to the fact that it has been pretreated. My only concern and fear is that a common complaint is that when painting on pre-fireproofed muslin that brushstrokes dry with a ring or salt stains occur around the edges? Is this true? Has anyone experienced this when painting on pre-fireproofed muslin? And if so was it so bad and noticeable that it ruined the painting?