The various forms of abrasives used by the painter for rubbing down or flatting work include the following:
The name ‘ sandpaper’ popularly applied to this material is a misnomer, for sand is not used in its manufacture, the grits consisting of powdered glass, flint, garnet, silicon carbide, and aluminous oxide, according to the type of glasspaper employed. These abrasive papers can be divided into two main classes – those for dry rubbing down and those for wet rubbing down. In the former, the grits are bonded to the backing paper with glue, which is easily affected by moisture. In waterproof abrasive papers, the bonding material is moisture-resisting and, as it impregnates the backing, makes the paper more flexible. Under the Lead Paint Regulations, dry rubbing down of painted surfaces is prohibited and thus, for this purpose, the use of dry glasspaper is ruled out. Waterproof abrasive paper, though more expensive than the other, is far more durable.
Ordinary glasspaper is sold by the quire or the ream (480 sheets) and is graded Nos. 00, o, 1, 1, F2, M2, S2, 2J, and 3, in increasing degrees of coarseness. Flint paper, which is a pale-yellowish colour, and has better cutting power than glasspaper, is graded Nos. 000, 00, o, J, 1, ij, and 2, the last named being the coarsest. They are available with both glue and waterproof adhesives.
Garnet paper is a reddish brown in colour and superior to flint. It is graded Nos. o (coarsest), 2/0, 3/0, 4/0, 5/0, 6/0, and 7/0. Silicon-carbide paper is dark grey to purplish black in colour. The grits are of extreme hardness but are, at the same time, brittle, the result being that when the paper is in use, minute particles of the grit split off, leaving always a new and keen cutting edge. Its grade Nos. are 60, 80, 120, 150, 180, 220, 240, 280, 320, and 400 (finest). Aluminous-oxide papers are a bright brown in colour and maintain a sharp cutting ability for a long time; they are graded in the same way as silicon-carbide papers.
Pumice Stone and Powder
This is an extremely porous, light-coloured volcanic rock, a kind of vitreous froth. It is ground into powder to form the basis of certain metal polishes. The stone itself is also dressed with smooth faces, and is then used for rubbing down hard paint, either in cleaning old painted surfaces, or in order to produce a smooth even surface. The stone is constantly dipped in water during the process of rubbing down. Pumice stone is also useful in smoothing wood after the use of sand or glasspaper, and for cleaning hands stained by pigments. It is derived chiefly from the Mediterranean, the best quality from the Lipari Islands, being ejected in the volcanic eruptions of Vesuvius and Stromboli.
There are various substitutes for pumice stone, in the shape of rubbing-down blocks which are made of silica and other cutting materials, formed in the shape of a block which can be used to rub down a surface with water in a very short time. These blocks are cheap and are extensively used. They are of varying degrees of fineness and save a great deal of time in actual work.
It may be mentioned here that in rubbing down varnish it is often found advisable to use, instead of powdered pumice stone, cuttle-fish bone, which is supplied in pieces assuming in shape that of the cuttle-fish. The bone is powdered and applied to the work with a piece of moistened cloth or a pad. In other words, the powdered cuttle-fish bone takes the place of powdered pumice stone, with the advantage that it does its work much quicker, being a sharper cutter.
This useful form of abrasive is not employed by decorators as widely as it deserves to be. It can be used either for dry or wet rubbing down and for smoothing, cleaning, and polishing purposes. Its chief advantage is that it adapts itself to the contours of the surface and thus is especially effective for mouldings, grooves, or turned work, where even the most flexible of glasspaper is liable to cut through at the edges.
Steel wool is made in nine grades: Nos. 000, 00, 0, 1, 2, 3, 4, 5 and 6; the finest of these, No. 000, can be used for polishing and is almost the equivalent of rottenstone.
Rottenstone is a special preparation of certain limestones which contain a heavy percentage of silica. They are mostly of organic origin. These limestones are broken up and steeped in water containing carbonic acid or humus acid, which causes the lime to dissolve, leaving the silica as a deposit in the form of a fine powder. This is dried, made up into bricks or otherwise treated, and is used in the preparation of polishes for metals, and also for rubbing-down purposes on various surfaces. Very good qualities are obtained from Derbyshire and South Wales, and prepared for the market in various forms,