Meanwhile, turn to the portion of the wall that you are going to plaster, making sure it is clean of any grease or smooth paint, and wet it down thoroughly with a mason’s brush or spray bottle. Notice how the wall absorbs the moisture. Wetting down is critical for adhesion because the water will transport the plaster into the base material so it becomes an integral part of it. (Old plaster walls, concrete block, soft brick, metal lath, and wood lath are all good bases for new lime plaster.) If the surface is painted, glazed tile or brick, or has insufficient suction, you cannot expect the plaster to bond. In this case, either chip the surface enough to produce a porous substrate or apply a plaster bonding agent. (Bonding agents are acrylic liquids sold in convenient quart cans or gallon jugs that you brush or roll onto the wall prior to applying plaster.)
For thick coats of plaster over masonry substrates or lath (the situation for most repairs), begin by troweling the lime-and-sand mix onto the wood lath, metal lath, or moistened wall. Lay the plaster on about 3/8″ thick, then notice how it behaves when you apply a wooden trowel to it. You will be able to rub the surface to flatten it without bringing water out, so you can form the plaster for rounded corners or around obstructions in the wall.
Always use a wood float to smooth and flatten the surface—not the steel trowel, which you should reserve for initial application and finishing operations. This is because if you over-trowel with a steel trowel while leveling or otherwise shaping the surface, excess water may migrate to the surface. This robs the plaster of the suction needed at the wall, and the new plaster will fall to the floor. If this happens, simply wait for the wall to dry somewhat and reapply the lime-and-sand mix.