The method of drawing out the design depends on the type of top coat. All plaster is exceedingly sensitive before it sets hard and retains indelibly any dust that falls on it or any lines used in blocking out the design. Fresco technique is, of course, based on this characteristic of plaster, but it has its dangers for sgrafflto. Scraped plaster is the most convenient surface to work on. The surface is not scraped until after the work is finished, and unwanted lines and marks can be eliminated by it.
What is often done is to make a full size cartoon of the outlines, lay it over the fresh plaster and press the lines through with a stylus, as for a fresco; but this is unnecessary. With sufficient experience of enlarging by squaring up, and a good feeling for the effects of engraving on a large scale, it is perfectly possible to work from a small drawing and draw directly onto the wall in freehand. It has the advantage that mistakes can be rectified at once, and sometimes an effect is very different on a wall from on paper. The freehand drawing does not lose spontaneity, as the traced one may well do.
Working on a surface which will later be scraped, it is possible to indicate the squares for enlarging onto the plaster by using string marks made with plumb line and level. If the top plaster is not to be touched afterwards, a lath frame can be hung up on hooks and a net of strong squares fixed to it. For rough cast the net must be hung on the scaffold so that it does not touch the plaster anywhere.
All the brass gravers and styluses and wire loops used by clay modelers can be used for cutting the lines and removing pattern for the design, but the author has found screw drivers and spatulas of different breadths, some with sloping ends, to be more satisfactory. Lines can be begun very thin with a sharp steel screw driver and then thickened by turning the implement towards the flat side. The thickness of the line can be varied as il would be with a broad drawing nib.
The outlines of the larger patches can be cut down first to just above the first sgraffito coat with a narrow, oblique spatula. They should be cut with a visible slope towards the surface. If the angle of the cut is too steep and is run along a ruler edge the effect is too metallic. All lines and outlines should be done freehand. It is better to make the patches too small at first rather than too big. It is easy enough to remove more plaster and make them bigger, but filling in is never wholly satisfactory and wastes a lot of precious time. The patches can be carefully removed with the flat of the spatula blade; generally the top layer comes away quite cleanly.
Wire loops take much longer to lift out the plaster, and this is irritating because of the time factor, which makes the artist hasty and nervous. With a little practice the spatula can be used very neatly. A loop can never come cleanly to the edges and into corners, so that in any case the work will always have to be touched up with the spatula.
At first, the top coat should be removed only deep enough for the sgraffito layer to show through. Thus, one quickly gets an over-all impression of the whole design and allows time for the freed surface to harden slowly. If the patches were scraped clean at once it would take longer to get an over-all impression of the design, and the damp under surface would get smeared, while the top coat was hardening in other places more than necessary to be worked. Thus, it is best to remove the top coat quickly allover, cnd then start on the final cleaning up.
The same method applies if there is more than one sgraffito layer. The whole design is first cut down to show up the first sgraffito layer. Then the surfaces that are to be in the second and subsequent colors are drawn into the first and all cut down nearly to the second. The process is continued for the third and any subsequent layers.
When all the lines and patches are cut, the plasterer will decide when it is time to scrape them clean, though in time the artist learns to judge for himself. If you use a spatula blade held perpendicular to the surface, the plaster should crumble off almost dry. If it smears it is not dry enough; the surface would form waves and too much of the colored layer would come away. No more should be scraped off than leaves the surface clean.
It is obvious that this must be done from the top downwards or dust would collect in what is already cleaned, and the lines would have to be cleared again. If the top coat is also to be scraped it should be left to the plasterer, who will do it together with the rest of the wall. Unless you are experienced, it is easy to miss patches on a large surface, which will show up lighter and spoil the whole facade. The sgraffito layers, however, should never be left to the plasterer, even for touching up, for his whole training compels him to straighten off all the edges.
The beginner can start practicing on a board. There are cemented, pressed boards made from wood pulp, which can be treated as a rendered wall. In a smaller format, it is possible without the plasterer's help to cover hardboard with a mixture of fine scouring sand, powder color, and a slow hardening gum of some sort; wallpaper paste is the best. Each coat should be about 12 inch thick. A pocket knife serves quite well as the cuting tool, both for scoring lines and cutting out patches.
The effect of this work can be rather like a mistaken "refinement" of the scraping technique, in which the finest residue of top layer is used to shade in a design on the undercoat. It is neither painterly nor graphic in effect and is entirely opposed both artistically and technically to sgraffito. The thin residue of plaster is very insecure and soon weathers away out-of-doors. It looks rather like a wall pastel, but, with its easy but pointlessly carved edges, carries none of the conviction of a pastel.