Installing stucco is pretty straightforward if you do enough research and have the right mixing ratios and application techniques but weather plays a factor too and some people ask whether or not you are able to apply stucco in the rain.
Can Stucco Be Applied When It Is Raining?
It is possible to apply stucco when it is raining out if you take certain extra measures. You will have to cover the walls you are working on, cover your mixing station and have an accelerator in the stucco material to get good results.
Covering The Walls:
The best possible way to ensure that the rain does not ruin your stucco walls is to cover the walls with some sort of material. Plastic works very well and I recommend using a thicker plastic like a 3 mil that way it will hold up and will be less likely to tear if a gust of wind comes by.
I usually just find a spot to attach the plastic to the fascia board and then run it down to the ground or scaffolding and weight it down or tie it off to something strong. Be careful though when attaching plastic to scaffolding because it can act as a giant sail and can pull over your scaffolding if it it not secured to the wall or ground.
I have also had good success with making a temporary shelter out of a few 2x4's and a couple of sheets of plywood, not OSB. This is a lot stronger and is not affected by the wing if built correctly. You can always add a layer of plastic to the plywood if you want to make sure that absolutely no rain gets onto the wall.
If you have high winds that are projected for your area (20 mph+) then a plastic covering will probably not be adequate for covering your walls. The plywood route is the better route.
Covering Your Mixing Station:
Your mixing station is just as important to cover up when it is raining because it can add a lot of unwanted water into the stucco base coat or finish coat material. It is also miserable working when the rain is getting you soaked and you can feel the added weight of your wet clothes getting heavier and heavier!
I usually just use an EZ UP type of canopy for my mixing station and it works out quite well. They have some on Amazon for around $100 but I have seen cheaper ones at places like Walmart and remember that it may get a bit dirty so don't use a super nice one that you don't want to get messed up.
If there is wind in the forecast, then making sure the pop up canopy is securely fastened to the ground is critical, as it can blow away pretty easily and in some cases, flip over and tear the canvas top, rendering it pretty much useless.
One thing to keep in mind when spreading material in the rain is the added humidity that will be in the air, which is usually close to 100% when it is raining! This added moisture will retard the setup time of cement based materials (base coats and cement based finish coats) so adding an accelerator to the material will help you out quite a bit.
Calcium Chloride is a good choice and I have also had pretty good luck with those accelerator packs that are sold at Home Depot that are primarily used for concrete but work well for nearly all cement based materials.
You could also use a warm water to mix up the stucco (warm, not hot) and this will also help speed up the set time. I usually use one of the submersible water heater elements because they are relatively cheap and can heat 5 gallons of water up in about 5-10 minutes.
Rain Affecting The Different Phases Of Stucco:
Lath: Rain doesn't really affect the lathing process all that much because the asphalt infused paper is water resistant and the wire is usually galvanized.
You do not want to apply the stucco paper over a saturated substrate though (plywood/OSB) as you will seal in the moisture and it will not dry out fast enough. Wait until the walls have had the time to dry out a bit and then apply the paper and wire.
Scratch/Brown Coats: Rain affects the base coats set time, like I mentioned earlier. In the summer, I can spread a wall and float it 60-90 minutes later, on average but in the rain that time can be extended to 120-180 minutes so the addition of an accelerator and/or warm water is essential in order to cut that time down.
Too much water on the wall (rain) after your base coat(s) are applied can result in the material sliding off of the wall
Finish Coats: The finish coat is a little harder to apply in the rain and you want to take extra precautions that no rain gets onto your finish material. This is the last coat of the stucco process and rain can distort the color and appearance of the final look of the stucco.