Identify the plaster and defect before choosing a repair
Cracking, detachment, staining and friability do not point to one universal repair. Historic England maps these symptoms to possible causes including moisture, movement, salts, inadequate key and incompatible material. Record the location, extent, substrate, coat build-up, moisture signs and movement evidence before reducing a defect to a filling item. Continuing water entry or possible structural movement needs separate assessment before decoration.