Including polishing and texturing.
What tools do I need to apply the plaster?
The same as for any other plastering job: a hawk, laying on trowel, detail trowels, plastic polishing trowel, fine mist sprayer, clean buckets for mixing, clean water, hand held plasterer’s whisk, sponges, masking tape, protective sheeting and appropriate respiratory protection.
Which tools for which coat?
The undercoat is applied with a steel trowel, and the top coat with a plastic trowel.
How do I apply the plaster?
Plaster is best applied using a hawk and steel ‘laying on’ trowel. Use broad, vertical strokes. Only cover as much surface as can be comfortably worked in a session. Do not overwork the plaster at the initial stage.
Once the entire wall has been covered, apply a thin skim coat using a wetter slurry mix, to remove any imperfections in the original plaster coat. Do this when the original coat is still workable, but has stiffened up slightly.
Once applied, the plaster should be left to set for 25 minutes (+/- will depend on room temperature/humidity and absorption rate of substrate). When to continue? A good indication is when the plaster stops moving under the trowel and the moisture shine on the surface of the plaster has begun to dull. You can then smooth away the trowel marks.
Clay plaster likes to dry naturally. Gentle air circulation is best. Keep external heat sources/strong draughts to a minimum.
Apply the base coat at 13-15mm thick, and the top coat 2-3mm.
Finishing: how do I get a smooth, polished effect? (more durable)
Use a plastic trowel. We do not recommend using a steel trowel for polishing, as the steel will oxidize on the plaster surface leaving black marks.
Do not polish until the plaster has sufficiently hardened (it does not move under the trowel). Once the plaster is stable, spray a small section of the wall with clean water (fine mist sprayer). Do not over-moisten the wall. Use firm, even strokes to work the wall with the plastic trowel. Move the trowel in all directions until a smooth, hard finish is achieved. The trowel should glide smoothly and cleanly over the surface of the plaster. Repeat this process until the whole wall has been polished.
Finishing: how do I get a textured effect? (easier on rough, undulating walls)
As an alternative to hard polishing, the plaster can be finished with a sponge. When the plaster has hardened sufficiently, work the plaster with a damp sponge. Use a circular motion. This will give a softer, grainy effect; as the sand particles are brought to the surface. Undulating walls such as old cob walls can be difficult to hard trowel. A tadelakt polishing stone can navigate the undulations. A sponge finish can also be used on straight walls, if this finish is preferred for aesthetic reasons. A note on corners: rounding them off makes them stronger and decreases the risk of damage. If hard corners are desired, avoid metal beading
Thanks to Clayworks for information.
