When filling (skimming or fine plastering), a compound is applied with a filling knife or trowel that can be sanded once dry. The goal is a smooth, level and crack-free surface as a basis for the further build-up. On a wall this includes filling plug holes, working over plasterboard joints and full-surface skimming (finishing levels in drywall). On a floor, filler or levelling compound evens out small height differences and unevenness in the screed.
A rough distinction is made between filler (for individual holes and cracks), surface filler (for full-surface skimming) and self-levelling compound in the floor area. Fine or finishing filler produces particularly smooth surfaces, as required for gloss coatings or walls sensitive to grazing light.
The process: clean the substrate and prime if necessary, mix the compound or apply it ready-to-use, pull it smooth in one or more layers, let it dry and then sand it. Two passes are often needed because the compound shrinks slightly as it dries. The sanded dust is removed, then priming and painting or covering follow.
Filling is always necessary when the substrate is damaged, uneven, open-pored or freshly plastered. The effort depends heavily on the condition: a few holes are done quickly, whereas full-surface skimming of grazing-light-proof walls is considerably more labour-intensive.
As a guide, simple patching and filling in Switzerland often costs CHF 10 to 25 per m²; full-surface skimming of demanding surfaces can cost more. Filling is frequently part of the painter's or floor layer's quote as preparatory work (guide values, no guarantee).