Plaster is a soft mixture of lime and sand. It is used in walls and ceilings as an alternative to drywall and joint compound. Forming a smooth, hard surface after it has dried, plaster is very similar to mortar or cement and is very hard to crack or damage. Joint compound is commonly installed over the seams between sheets of drywall. When the plaster on your wall does crack, it can be repaired using a mixture of plaster and joint compound. This mixture will expand to fill the joints when it is applied.
Locate the crack or spot on the wall that needs to be repaired and determine how much joint compound and plaster is needed for installation. If the cracks are small, only one bag of each needs to be mixed.
Fill a large bucket 1/4 full of water and pour in the bag of joint compound. Mix the two together with a trowel. Stir around in a circular motion along the edges and get in the middle also. Move the compound around until the mixture is a pasty consistency. Add more water if necessary.
Pour in the plaster powder. Dump in twice the amount of plaster as joint compound and continue mixing with the trowel. Turn the mix with the trowel in every direction and make sure there is a pasty consistency like peanut butter throughout. Add more water as necessary until there are no dry spots in the bucket.
Remove the trowel and wait 5 minutes until the mix is solid enough to work with. Apply the mix quickly before it sets.
The mixture should consist of 2/3 plaster and 1/3 joint compound.