the person that told you that is wrong u just go ahead and paint it, the paint will not flake. the only reason u water down paint and put that on first is when you have a wall plastered indoors and you give it a white wash to show up any blemishes that need to be sanded down.
make sure you put fibers in the mix as well to prevent any cracks showing up after and i would paint it once it has all gone off as the paint holds the stones together allot better