I am replastering internally a listed building and have to use lime mortar plaster. I have to use washed sand mix 3 to 1 with hydraulic lime. The builders merchant have sent me a large bag of sand with a 6 to 1 mix of a reddish sand which appears to be building sand and they say it is ok to plaster with. My understanding is that that building sand is red because it has clay in it which would make it very difficult to trowell onto a wall.Am I right or wrong.