Stucco or plaster (we'll use the terms interchangeably herein) is made by mixing sand and portland cement in various ratios to create base, middle and finish coats. And, although you can mix your own, we suggest that you purchase ready-mix patch it's easier. Modern stucco is applied in five layers. You will not have as many steps with patchwork, but you should know that there is a difference between a patch and a new installation.
With wood framing, stucco is always applied over a thickness of building paper such as asphalt-impregnated felt. Next, a layer of chicken wire known as wire lath is attached with spacer nails to hold it about 1/4 inch away from the paper. The paper acts as a waterproof barrier and the lath grasps the plaster. Three individual coats of plaster then are applied. By adding the plaster in layers the chance of cracking is greatly reduced. Deep scratches are etched into the surface of the first coat the scratch coat. The long narrow indentations and other surface irregularities help to ensure a bond between the first and second coats. Both coats are gray in color, and when dry, resemble plain old-fashioned mortar or grout.
The final coat contains a dye and, therefore, is known as the color coat. Patching plaster is not nearly as difficult as a first-time installation. This is mainly true because small areas do not expand and contract in the same way that larger ones do. Three coats for large areas; one or two for small ones.
So what is so difficult about mixing sand and cement and troweling it into an opening the size of a baseball? Nothing really. You need only be sure that there is a ragged base to bond the patch mix to what exists. If the wire and paper are showing, it is important that there are no holes in the paper. There are several good sealants available that can be applied with a caulking gun. All loose chunks of old mortar, and powder, too, should be removed from the repair area. Also, the wire lath should be pulled away from the paper far enough so that the patch material can ooze in between them.
You can make your patch in three coats if you wish, but we suggest no more than two applications. Also, we don't recommend trying to match color coats. It is virtually impossible to get two new coats of plaster to match, let alone an old coat and new one.
It is much easier to apply the first two coats as one layer and a second coat with no color. Once cured, it can be painted to match exactly. When applying the first coat, make sure to leave about a 1/4-inch recess. The final coat doesn't have to be as thick. Although normal practice is to wait seven days between coats for proper curing, with patchwork the second coat can be applied as soon as the first layer dries. By the way, the consistency of the patch material should be somewhere between pancake batter and bread dough. That is, it should be thick enough to stick to the wall yet thin enough to manipulate with a trowel.
Exterior plaster usually is textured. Textured finishes are less expensive than smooth surfaces because the texture hides flaws. You may have to finish your patch with a texture to match the surrounding area. There are two common textures: dashed and skip-troweled. Another word for dashed is "splattered" so called because the finish can be splattered on with a paint brush. Thin down a batch of the finish material, and, holding a board or a broomstick as a barrier between yourself and the wall, strike the paint brush against the barrier. For a skip-troweled finish, add a cup of sand to the finish mixture instead of thinning it. The sand will cause the material to stick randomly and miss or "skip" areas as it is being applied with a trowel.