How to Fix Textured Plaster Walls

Reattach loose plaster to the lath before you move another step further. This can be done with plaster screws and will prevent the damage from spreading. Failing to do this step will create a temporary fix at best because the cracks are a symptom of the loose plaster. There will only be more cracks in the future if you don't fix this problem first.

Widen the crack a bit so that you can clean the lath, removing dust and debris, to provide a surface that the plaster can bond to. It is also a good idea to hollow out the space a bit so that there is an area of salvageable plaster that extends over the gap, creating a lip to hold in the patching plaster.

Use a sponge to slightly moisten the area and then fill it half full with patching plaster. A putty knife is an excellent tool for this job. Once this plaster has hardened to a tacky thickness you will want to score it so that the next layer of putty will stick to it and then allow it to dry completely.

Moisten once again with a damp cloth or sponge and then add more patching plaster. Be careful that you do not completely fill the gap as you still need to add a layer of finishing plaster.

Allow to dry completely and then add a layer of finishing plaster. You want to avoid overfilling the gap as the excess will need to be sanded down, but be careful not to leave a noticeable dip in the wall either. Allow the finishing plaster to dry.

Match the texture of your existing plaster walls by either purchasing spray on plaster that is designed to match or imitating the texture with tools you have around the house. It doesn't necessarily need to be an exact match but needs to be close enough that it will blend in with the texture of the wall quite nicely.