The Technique of FURNITURE MAKING
SANDING PAPERS
Grits for sanding are deposited on the backing support either by gravity or in an electrostatic field in order that each individual grain shall be secured in an upright position, giving maximum cutting performance to the teeth. Figure 32 shows the disposition of the grains where A is the backing, B the basic adhesive coat, usually of hide glue, C the second adhesive coat of hide glue or synthetic resin and D the abrasive grain. The use of hide glue for the basic coat imparts a certain degree of flexibility to the sanding sheet, while the synthetic resin in the second coat anchors the grain in an upright position and does not soften under the heat generated by the cutting action. Generally speaking, grits anchored entirely by hide glues cut softer because the heat softens the glue, permitting the upright grain to lean over slightly, while the use of resin glues throughout gives a harder, fiercer cutting action, and will withstand the considerable degree of heat generated; therefore manufacturers use either one or a combination of both, according to the type of abrasive material and the purpose for which it is to be employed. Waterproof papers for wet sanding are anchored exclusively with synthetic resin glues.
The types of abrasive material available for wood finishing are as follows:
Glass
Originally crushed sand was used, hence the familiar name 'sandpaper', but the grit is now powdered glass, preferably from old gin or beer bottles. Slow cutting, relatively soft and soon exhausted glass is mainly used for cutting down old paint-work, although it is also used for wood finishing as it does not scratch. The flour grade 00 is still a favourite with french-polishers.
Flint
A natural abrasive used for hand-sanding and only slightly harder than glass, it has now been largely superseded by faster materials; moreover flint-dust embedded in the grain of sanded surfaces (cheap plywoods, etc.) has been found to cause a chemical reaction with synthetic finishes.
Garnet
Crushed garnet is used almost entirely for sanding wood. It is much harder than glass or flint, cuts quicker, lasts longer, generates very little heat and consequently rarely burns the wood. It remains a favourite material among cabinet-makers, as it yields a very smooth finish.
Aluminous oxide (Aloxite)
An impure oxide of aluminium manufactured from bauxite; the purest forms yield sapphires. rubies and Oriental topaz. The toughness and hardness of this material ensure that the individual grains will not disintegrate under the considerable mechanical strains of high sanding pressures; therefore it can be used to cut harder and faster than other materials. The white is the purest form and is usually dyed orange and reserved for wood finishing, with the impure forms of brown and pink for metals, etc. Occasionally these coarse-cutting impure forms are dyed black.
