Exterior plaster, commonly known as stucco, is a concrete derived material created to withstand the rigours of external existence. Most of the plaster can last for years with only a small amount of maintenance required to keep it in good condition. Even the best of an exterior plaster coating can develop cracks though, and when it does, you'll need to conduct repairs as soon as possible to prevent the cracks from growing. A repairable crack requires little more than new mortar, a covering of fibreglass tape and a concealing paintjob. Done well, the newly repaired crack should be indistinguishable from the surrounding surface and as strong as the original plaster coating.
Remove any loose plaster and then open the plaster crack slightly with a carbide tipped scoring tool. Open the crack to the wood lath surface, taking care not to cut into the lath itself. Widen the crack so that its slightly wider at the base than at the top, giving the patch a ledge to stick to. Remove any dust created from opening the crack with a shop-vac to clear the crack for the replacement plaster. Soak the plaster with water to prevent old plaster drawing moisture from the repair materials.
Mix a repair mortar using one part Portland cement to four parts builder's sand. Create the mix in a medium-size bucket using an electric drill with paddle-bit attachment. Add water to the mix until it's the consistency of mayonnaise. Fill the cracks with the mortar using a 6-inch taping knife and apply the compound to the crack and nearby surrounding area to a 1/8-inch depth.
Apply a strip of bitumen-coated fibreglass onto the crack, embedding it into the mortar. Cut the fibreglass with a utility knife when needed in order to apply only straight strips of the material over the crack. Cut the fibreglass into pieces rather than curving it, which can cause wrinkles. Scrape the surface of the fibreglass with the knife to remove excess mortar, leaving the material covered in a light mortar layer. Wait about an hour for the mortar to begin to set.
Cover the fibreglass with a second coat of mortar, brushing it over the fibreglass to match the pattern of the surrounding plaster. Allow the mortar to dry overnight.
Prime the patched surface with a stucco primer, using a paintbrush for the primer application. Allow the primer to dry completely for about 12 hours then cover with an exterior latex paint to match the surrounding plaster surface. After drying, the paint will conceal the patched crack, blending the patch in with the surrounding plaster surface.
Protect your eyes and lungs from stucco and mortar dust by wearing safety goggles and a respirator during the repair process.