This blog entry gets a fair number of hits. As an FYI, there are photos and a bit more written text in my weekend update from 22 Sep, 08.
I've modified the text below from instructions first posted on the Better Homes and Gardens website. I got really confused about a month ago when my stucco guy found my prep work completely lacking. I had prepped expanded metal mesh over house paper, which allows quite a bit of flex in the EMM. He wanted a firm substrate to work with, and I've since found at least one other web site that stated there are regional differences in stucco techinques: some regions do metal mesh over paper, others back that with plywood sheathing. The advantage of plywood sheathing is that you don't have to wait for the first scratch coat of stucco to dry and you can finish a wall in one day as a result. With EMM over paper, the scratch coat (when dry) provides the rigid substrate.
Here's my edit of the BHG article. Their's had nice graphics, but I don't think they're necessary for an understanding of what to do.
Of all wall materials, stucco is one of the most difficult to patch. Small cracks can be widened slightly and filled with patching cement. But if the damage is more extensive, you must chip away the old material down to the lath or masonry underneath, then build up a new surface in three layers.
Problem areas larger than 4 feet square usually require that the entire wall be restuccoed -- a job for an experienced masonry contractor (or a poor and desparate homewoner). Small, do-it-yourself patching jobs can be done during mild weather when there's no danger of freezing. Plan on the project taking at least three days. Wait at least six weeks before painting, then prime and paint with a concrete coating. Note: I'm not planning on a concrete coating at this opint. This may be intended for areas with more rain or snow.
Colored stucco is difficult to match. Experiment with pigments, keeping in mind that colors will fade as much as 70 percent by the time the stucco dries. Never let the coloring pigment exceed 3 percent of the batch's total volume.
3 working days for a large patch extended over 10 to 12 days to cover drying time. (Unless stuccoing over plywood / paper / EMM in which case you needn't wait for drying)
Moderate to advanced masonry skills. (If you watch someone do this once or twice, you'll have the confidence to tackle this yourself, or have a sufficiently high level of poverty+desperation.)
Cold chisel, pry bar, hammer, stapler, trowel, hawk, improvised rake, spray hose, metal straightedge.
1. Prepare the area
Wearing gloves and goggles, chip away loose stucco with a hammer, cold chisel, and pry bar. Use snips or wire cutters to cut metal lath. Staple new roofing felt and metal lath onto sheathing. Make sure the surrounding stucco is firmly attached; if not, chip away more.
1A. If repairing a large area, feather / bevel the edges of the remaining stucco with the chisel so that you can blend in the new stucco better. If repairing a smaller area (a crack, for instance). Some practitioners recommend chiseling back until you've revealed the original underlying chicken wire / EMM in order to tie the repair in physically with original. This is to prevent cracks along the new/old interface. Adhesion promoter (step 2A) eliminates this need, in theory.
2A. Apply concrete adhesion promoter to edges of existing stucco. A 1 gallon container is about $10 at Home Depot, and that ought to be a lifetime supply - it appears to be something like carpenters wood glue, and indeed CWG might work in a pinch. This is essential in areas around windows and doors, where there is high likelihood of cracks propogating along the interface between new and old stucco. My buddy Warren was told that stucco repair caulk would also do the trick, but I don't see how a bead of compliant caulk will hold anything past the first time the door is slammed.
2B. Apply the first coat
Apply the first coat of stucco to a depth about 1/4 inch below surrounding surfaces. Press with the trowel to embed it firmly in the lath. My stucco guy used a VERY thin mix of stucco over plywood / paper / EMM. If working with only paper /EMM it will need to be thicker.
3. Rake the patch if applying over paper / EMM.
When the stucco begins to firm up, scratch it with an improvised rake made by driving nails through a piece of wood. The scratches should be about 1/8-inch deep.
3A. Rough the patch surface using a wooden float if applying over plywood / paper / EMM.
4. Mist the patch
Mist the scratch coat with fine spray, keeping it damp for two days. In windy or sunny weather, repeat several times daily. I bought a spray bottle for this purpose. I think a hose would be too uncontrolable.
4A. Obviously, you don't wait if stuccoing over plywood / paper / EMM, but do keep a spray bottle on hand for misting. Use as needed.
5. Apply the second coat
Apply the second (brown) coat to within about 1/8 inch of the surface and level it off with a straightedge.
6. Float the second coat
"Float" the brown coat by working it with a trowel until water comes to the surface.
6A. Rough the coat with the wooden float if applying over plywood / paper / EMM.
7. Apply the finish / color coat (so-called even if it doesn't have color).
Mist the brown coat for two days, then wait a week. While you are waiting, practice applying finish stucco to a piece of plywood until you can achieve a texture that blends in with the rest of the wall. To complete the job, moisten the brown coat and mix a batch of finish stucco. Smooth on finish coat. Texture it within half an hour.
7A. If stuccoing over plywood / paper / EMM wait for the stucco to dry sufficiently to float it, drawing water to the surface. Texture it to match surrounding stucco at this time.