It is vital to use a scarifier when plastering a wall--without it, the plaster will loosen and crack as it dries. Lumps may even fall out, ruining the smooth finish. A scarifier is a comb-like tool, with a set of long, thin metal teeth. After an underlying layer of plaster has been applied to a wall, the plasterer scratches the wet surface all over with the scarifier. The diagonal scratches run left to right and right to left, creating a cross-hatched appearance. The rough surface of this "scratch coat" allows the top coat of plaster to "key" into it when applied, bonding perfectly to produce a smooth, unblemished result. A homemade scarifier is a simple and quick tool to assemble.
Rub the block of wood vigorously with sandpaper, to remove any splinters. You will grip this block of wood hard when using the scarifier, so it needs to be splinter free.
Draw a line with the pencil along one 12-inch side of the block, exactly 1 inch from the block's edge.
Measure and mark off 1-inch divisions along the line you have drawn. There will be 11 of these marks in total.
Clamp the block in the vice of a workbench, with your marked line uppermost.
Drive a round wire nail into the block at each of the 11 points you have marked, using a hammer. Drive the head of each nail fully home, so that the points of the nails stick out on the opposite site of the block, to form the teeth of the scarifier.
Remove the block from the vice and grip it in your palm with the points facing outward. The scarifier you have made is ready for use.
If you are new to plastering, practice applying layers of plaster to an old 2-foot-by-2-foot square of plywood, before tackling a wall.