natrosol gets thicker the longer you leave it, although it mixes to really thick in about 2-3 minutes depending on how much powder you have. But generally gets even thicker if you leave it.
Its always best to mix gunge at "Bath Temperature" mixing it with water that is too hot can be problematic in terms of the time for the gunge to thicken. Adding more powder to the gunge after it has been mixed can also cause clumping and doesn't usually work. You can mix up a thicker batch in another bucket then add that to the thin gunge though. When we mix the Dunk Tank Gunge we have done it like that before.