Measuring, Setting Out and Preparing Backgrounds and Surfaces
Preparation decides the finish
Most plaster failures trace back to preparation: dusty, friable, too absorbent or too smooth backgrounds, or missing keys and beads. This domain tests measurement, setting out datums and lines, and the preparation of masonry, concrete, plasterboard and existing plaster.
Core concepts to know
- Measuring and setting out: use datum lines and levels to establish screeds and rules; check square with the 3-4-5 method; set out bead lines and screed positions before applying material.
- Background preparation: remove loose material, dust, grease and paint; brush down; dampen absorbent masonry to control suction; apply bonding agents or spatterdash where specified.
- Keys and suction: a good key holds the plaster mechanically; controlled suction stops the mix drying too fast. Very smooth concrete needs a bonding treatment or mechanical key.
- Plasterboard: fix securely, stagger joints, tape and fill joints, and use the correct board for wet areas.
- Beads and stops: fix corner, stop and movement beads plumb and level, with the correct fixing spacing.
- Existing plaster: assess soundness; cut back to sound material and patch with a compatible mix.
Traps
A frequent trap is adding water to a mix to make it easier to spread on a dry background rather than dampening the wall. Another is fixing beads by eye rather than to a line or level, or assuming paint must be removed only when it is flaking.
Worked rule of thumb
Before the first coat, run a simple checklist: sound, clean, keyed, and suction controlled. If any of the four is missing, stop and fix it - no finish coat will compensate for a bad background.