I just did my own mix of 1/2 part NHL 3.5, 1/2 part lime putty, 1 part fine hemp and 1 part sand as my 1 cm thick backing coat on the back of my cork boards. I primed the wall itself with a sticky pure lime mix immediately prior to mushing the coated cork boards onto the wall. Adhesion seems good 1 week later. I don’t think that isovit stuff is necessary.