To apply roughcast to an exterior wall, clean and repair the base render if necessary, apply an undercoat and then add the topcoat with the roughcast mix. Wait for a few days of good weather so the new roughcast can dry properly.
- Clean and repair the wall
When roughcasting a wall, first fill small cracks with cellulose or vinyl-based filler and larger cracks with repair mortar. Strip off old paint or loose plaster. Treat moss or fungal growth with a solution of one part bleach to 10 parts water, and allow it to dry.
- Apply the base
Prepare the base, or undercoat, by mixing lime with water and allowing it to hydrate, or slake, for 24 hours. Follow the manufacturer's instructions for the correct amounts of lime and water. Afterwards, mix one part lime, one part cement and six parts sand. Apply this mixture evenly on the wall with a trowel, and then use a notched trowel to create a grooved surface so the topcoat of roughcast adheres properly. Let the base dry for 24 hours.
- Apply the roughcast
For the topcoat, mix one part lime, one part cement and five parts sand. To this mortar add small pebbles, shells or gravel. Apply the topcoat evenly on the wall. If there are any inconsistencies in the roughcast, use gloved hands to apply more roughcast material to the wall. Alternatively, to use the roughcasting technique known as pebbledash, apply the topcoat without the roughcast material. Spread plastic sheeting below the wall. Use a scoop or trowel to cast loose pebbles at the wall so that they stick to the topcoat. Afterwards, press the pebbles lightly into the mortar with the trowel.