Mortar mix is used to adhere stone or stone veneer to itself or to another surface. Mortar mix is rock-hard when dry, but can crumble and disintegrate with age. Like glue, mortar forms a bond with whatever surface it is applied to. When mortar is applied directly over paint or sealers, it forms a bond with the paint or sealer, not with the surface underneath. This can cause the mortar to fall away over time.
Does Painted Concrete Hold Mortar?
Mortar cannot be installed directly over painted concrete itself. First, a surface more textured and solid must be applied over the painted concrete. This surface is called a scratch coat. A scratch coat will give the mortar something to hold on to, instead of the layer of paint over concrete.
Before you apply the scratch coat, you must first put in place a system for appropriate ventilation during times of rain and moisture. Install a breathable grade D paper, then install a rainscreen or drainable housewrap, both of which are meant to provide a means for rainwater to run down to the lowest point of the wall and exit. This prevents moisture from damaging the structural part of the wall. Follow the manufacturer's instructions for installation.
Metal lath is installed over the rainscreen and paper, using the appropriate metal fasteners. Metal lath is like a screen of metal mesh that will provide something for the mortar to grip onto. The edges of the lath are overlapped by approximately 1 inch. Sheets of lath installed higher up on the wall are centered above the lower sheets of lath, so that the sheets are staggered like bricks on a wall. After the lath is installed on the wall, mortar is mixed and scraped onto the metal lath with a trowel. When the mortar dries, this becomes the scratch coat for the next layer of mortar to grip onto.
Now the wall is ready for a layer of mortar that will adhere to the scratch coat. If you're using the mortar to hold on stone veneer, the mortar should be scraped onto the back of the stone and then pressed into the scratch coat. The mortar that squishes out over the stone will then form the mortar joint between that stone and the next stone.