Grattage (~scratching/nailboarding)

Description

The grattage - or finition grattée - (Scratching – scratch or nail board finish - ) gives a rough texture, more or less determined by the aggregates contained in the plaster.
As the plaster begins to set you can start by grattage with the nail board. If the plaster clings to the nails of the nail board, stop, wait giving opportunity for the plaster to set further before trying again.
Check for bulges on the surface with the rule and eliminate them.
Make sure the entire surface is gratté (scratched). Otherwise, non-gratté (uncut) portions will dry significantly lighter often results in a blotchy, unsightly finish that requires a sanding blasting of the entire surface to eliminate.

Select a nail board with nails appropriately fine to the aggregate in the plaster and the fineness of finish desired.
The later the grattage is done as the plaster sets harder the smoother will be the surface between the grains. To obtain an even finish scrape the surface deeply.