First and foremost, prevention is key! Keep water out.
Think of it like putting a raincoat on the plaster rather than waiting for it to get wet and then trying to dry it.
After the gypsum board is installed, it's essential to apply putty and paint-preferably with a waterproof latex paint. Paints designed for kitchens and bathrooms are often the most effective. This protective layer acts like a raincoat for the plaster. Even if the air becomes humid or a splash of water lands on the surface, as long as you wipe it off promptly, the plaster underneath will remain safe.
In damp spaces with high humidity and water splashing around, do not use regular gypsum board, it should be waterproof gypsum board (or moisture-resistant gypsum board). The various types of waterproof gypsum boards include a waterproofing agent in the core and have coated the outside with a green water-resistant paper, giving it at least a little more than the standard gypsum. Regardless of the waterproof gypsum board used, I suggest covering it with any waterproof paint for extra protection, and for peace of mind.
While tiling an area, in a shower for example, the most common scenario is waterproof paint plus tiles. Before the installation of the tiles, always apply a professional waterproof coating (for example, flexible waterproof mortar) to the drywall or cement based products that are utilized as a base, making sure to coat every corner. In showers I like to apply the waterproofing product up to a height of 1.8 meters to ensure complete waterproofing. Even if water and moisture get past the tile joints, it will not get to the gypsum board in this scenario.
What to do if water has accidentally gone on the surface?
Immediately blot with a dry cloth: Take a dry absorbent cloth or towel and absorb the water from the surface. Only blot the water, do not rub as rubbing can spread the water further into the material.
Utilize a hairdryer (with caution): Adjust the setting to cold or low heat, keep a reasonable distance, and give a gentle blow to the wet spot. Never use high heat as a rapid change in temperature can separate the plaster and cause deformation or cracking.
Watch patiently for several days after drying: After drying the exposed water, watch the surface for several days. Look for blisters, yellow water stains, and bulging putty. If the affected area is tiny and the putty is only mildly yellowed, you may only have to sand thoroughly and repaint the area. If the affected area is large, is soft to the touch, or has black mold spots, you will need to cut out that area and add new gypsum board because the surface structure is irrevocably compromised.
Everyday Care
Ensure your home has good ventilation and remains dry. Don't leave plaster walls wet for extended periods.
During cleaning, do not wet mop up against the wall.
When hanging decoration or a picture, try to avoid nailing into potential plumbing locations. A leak in the wall could damage area.