Before plastering a surface, always apply a plaster undercoat or a mortar rendering to it. Make sure to check with the supplier that the undercoat is the correct type for the plaster.
When applying- a plaster undercoat to a small area use a wooden float and work in thin layers of plaster – about 3mm 1 in thick at a time. Build these up to a maximum thickness of about 12 mm though this will depend on the type of plaster being used. Never try to apply this thickness in one operation: this will cause the plaster to fall from the surface under its own weight as fast as it is applied. On larger areas work between two 12 mm thick uprights and level the plaster with a straight-edge pressed against the uprights.
When using a mortar mix. Make the mix weak – one part of cement. One part of lime and six parts of sand is perfectly adequate. A stronger mix will prevent moisture being absorbed and lead to damp patches on the wall.
Before the undercoat dries. Roughen up the surface slightly with a scratcher. This provides a key for the final coat.