I want to do a flagstone patio on top of an existing concrete patio. Because of the door threshold, I only have 2", at the most, to work with. I have access to some very nice brown flagstone that is under 1" thick. My plan is to clean the patio and then use type S mortar at a 2 : 1 ratio --- sand to mortar. Should I add an acrylic additive to the mortar mix to improve bonding or can I go with the mortar mix exclusively? I hesitate to use the acrylic additive because if it gets on the stone, it is extremely hard to clean off. You can see it on the stone. Any thoughts would be greatly appreciated.