Spray painting is a painting technique where a device sprays a coating (paint, ink, varnish, etc.) through the air onto a surface. The most common types employ compressed gas—usually air—to atomize and direct the paint particles. Spray guns evolved from airbrushes, and the two are usually distinguished by their size and the size of the spray pattern they produce. Airbrushes are hand-held and used instead of a brush for detailed work such as photo retouching, painting nails or fine art. Air gun spraying uses equipment that is generally larger. It is typically used for covering large surfaces with an even coating of liquid. Spray guns can be either automated or hand-held and have interchangeable heads to allow for different spray patterns.
An air compressor is a device that converts power (usually from an electric or diesel or gasoline engine) into kinetic energy by pressurizing and compressing air, which is then released in quick bursts. There are numerous methods of air compression, divided into either positive-displacement or negative-displacement types.
Positive-displacement air compressors work by forcing air into a chamber whose volume is reduced to effect the compression. Piston-type air compressors use this principle by pumping air into an air chamber through the use of the constant motion of pistons. They use unidirectional valves to guide air into a chamber, where the air is compressed. Rotary screw compressors also use positive-displacement compression by matching two helical screws that, when turned, guide air into a chamber, the volume of which is reduced as the screws turn. Vane compressors use a slotted rotor with varied blade placement to guide air into a chamber and compress the volume.
Negative-displacement air compressors include centrifugal compressors. These devices use centrifugal force generated by a spinning impeller to accelerate and then decelerate captured air, which pressurizes it.
The air compressors seen by the public are used in 5 main applications:
- To supply a high-pressure clean air to fill gas cylinders
- To supply a moderate-pressure clean air to supply air to a submerged surface supplied diver
- To supply a large amount of moderate-pressure air to power pneumatic tools
- For filling tires
- To produce large volumes of moderate-pressure air for macroscopic industrial processes (such as oxidation for petroleum coking or cement plant bag house purge systems).
Most air compressors are either reciprocating piston type or rotary vane or rotary screw. Centrifugal compressors are common in very large applications. There are two main types of air compressor’s pumps: Oil lubed and oil-less. The oil-less system has more technical development, but they are more expensive, louder and last for less time than the oiled lube pumps. However, the air delivered has better quality.
A sanding block is a block used to hold sandpaper. In its simplest form, it is a block of wood or cork with one smooth flat side. The user wraps the sandpaper around the block, and holds it in place (by inserting a fitted piece of cardboard under the sandpaper, one can soften the impact on the wood and protect against tears or uneven wear on the sandpaper). Sanding blocks are helpful because they prevent the “waves” created by plain sandpaper.
Fancier versions use clips, teeth or clamps to hold the paper in place. Commercial versions can be constructed of various materials. They are usually sized to hold a quarter or half sheet of sandpaper. Some versions use the sandpaper belts intended for a power belt sander. Construction workers often use commercial one-piece sanding blocks consisting of a foam plastic block with an abrasive coating.
A card scraper (also known as a cabinet scraper) is a woodworking shaping and finishing tool. It is used to manually remove small amounts of material and excels in tricky grain areas where hand planes would cause tear out. Card scrapers are most suitable for working with hardwoods, and can be used instead of sandpaper. Scraping produces a cleaner surface than sanding; it does not clog the pores of the wood with dust, and does not leave a fuzz of torn fibers, as even the finest abrasives will do.
A plane is a tool for shaping wood. Planes are used to flatten, reduce the thickness of, and impart a smooth surface to a rough piece of lumber or timber. Planing is used to produce horizontal, vertical, or inclined flat surfaces on workpieces usually too large for shaping. Special types of planes are designed to cut joints or decorative moldings.
Hand planes are generally the combination of a cutting edge, such as a sharpened metal plate, attached to a firm body, that when moved over a wood surface, take up relatively uniform shavings, by nature of the body riding on the ‘high spots’ in the wood, and also by providing a relatively constant angle to the cutting edge, render the planed surface very smooth. A cutter which extends below the bottom surface, or sole, of the plane slices off shavings of wood. A large, flat sole on a plane guides the cutter to remove only the highest parts of an imperfect surface, until, after several passes, the surface is flat and smooth.
Though most planes are pushed across a piece of wood, holding it with one or both hands, Japanese planes are pulled toward the body, not pushed away.
Woodworking machinery that perform the same function as hand planes include the jointer and the thickness planer, also called a thicknesser.
Wood putty, also called plastic wood, is a substance used to fill imperfections, such as nail holes, in wood prior to finishing. It is often composed of wood dust combined with a binder that dries and a diluent (thinner), and, sometimes, pigment. Pore fillers used for large flat surfaces such as floors or table tops generally contain silica instead of or in addition to wood dust. Pores can also be filled using multiple coats of the final finish rather than a pore filler.
The main problem in using putty is matching the colour of the putty to that of the wood. Putties are usually sanded after they dry before applying the finish.
Many different brands, types and colours are commercially available. Binders include lacquer, water-base, and linseed oil. Some woodworkers make their own putty using fine sanding dust (not sawdust which is too coarse) with wood glue or a wood finish such as shellac.
A grain filler or paste wood filler is a woodworking product that is used to achieve a smooth-textured wood finish by filling pores in the wood grain. It is used particularly on open grained woods such as oak, mahogany and walnut where building up multiple layers of standard wood finish is ineffective or impractical.
A wood stain consists of a colorant suspended or dissolved in a ‘vehicle’ or solvent. The suspension agent can be water, alcohol, petroleum distillate, or the actual finishing agent (shellac, lacquer, varnish, polyurethane, etc.). Colored or ‘stained’ finishes, like polyurethane, do not penetrate the pores of the wood to any significant degree and will disappear when the finish itself deteriorates or is removed intentionally.
Two types of colourants are used, pigments and dyes. The difference is in the size of the particles. Dyes are microscopic crystals that dissolve in the vehicle and pigments are suspended in the vehicle and are much larger. Dyes will color very fine grained wood, like cherry or maple, which pigments will not. Those fine-grained woods have pores too small for pigments to attach themselves to. Pigments contain a binder to help attach themselves to the wood.
The type of stain will either accentuate or obscure the wood grain and neither is superior to the other. Most commercial stains contain both dye and pigment and the degree to which they stain the appropriate wood is mostly dependent on the length of time they are left on the wood. Pigments, regardless of the suspension agent, will not give much color to very dense woods but will deeply color woods with large pores (e.g. pine). Dyes are translucent and pigments are opaque.
Gel stains are more akin to paint and have little penetrating ability.
Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand. It is processed and sold as dry flakes (pictured at right), which are dissolved in ethyl alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish. Shellac functions as a tough all-natural primer, sanding sealant, tannin-blocker, odour-blocker, stain, and high-gloss varnish. Shellac was once used in electrical applications as it possesses good insulation qualities and it seals out moisture.
A drying oil is an oil that hardens to a tough, solid film after a period of exposure to air. The oil hardens through a chemical reaction in which the components crosslink by the action of oxygen (not through the evaporation of water or other solvents). Drying oils are a key component of oil paint and some varnishes. Some commonly used drying oils include linseed (flax seed) oil, tung oil, poppy seed oil, perilla oil, and walnut oil. Their use has declined over the past several decades as they are replaced by alkyd resins and other binders. Oils that are susceptible to drying are also susceptible to becoming rancid autoxidation, the process by which fatty foods develop off-flavors.
In a general sense, lacquer is a clear or coloured varnish that dries by solvent evaporation and often a curing process as well that produces a hard, durable finish, in any sheen level from ultra matte to high gloss and that can be further polished as required.
The term lacquer originates from the Portuguese word for lac, a type of resin excreted from certain insects. Regardless, in modern usage, lac-based varnishes are referred to as shellac, while lacquer refers to other polymers dissolved in volatile organic compounds (VOCs), such as nitrocellulose, and later acrylic compounds dissolved in lacquer thinner, a mixture of several solvents typically containing butyl acetate and xylene or toluene.
While both lacquer and shellac are traditional finishes, lacquer is more durable than shellac.
Varnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss sheens by the addition of “flatting” agents. Varnish has little or no colour, is transparent, and has no added pigment, as opposed to paints or wood stains, which contain pigment and generally range from opaque to translucent. Varnishes are also applied over wood stains as a final step to achieve a film for gloss and protection. Some products are marketed as a combined stain and varnish.
After being applied, the film-forming substances in varnishes either harden directly, as soon as the solvent has fully evaporated, or harden after evaporation of the solvent through certain curing processes, primarily chemical reaction between oils and oxygen from the air (autoxidation) and chemical reactions between components of the varnish.
Resin varnishes “dry” by evaporation of the solvent and harden almost immediately upon drying. Acrylic and waterborne varnishes “dry” upon evaporation of the water but experience an extended curing period. Oil, polyurethane, and epoxy varnishes remain liquid even after evaporation of the solvent but quickly begin to cure, undergoing successive stages from liquid or syrupy, to tacky or sticky, to dry gummy, to “dry to the touch”, to hard.
Environmental factors such as heat and humidity play a very large role in the drying and curing times of varnishes. In classic varnish the cure rate depends on the type of oil used and, to some extent, on the ratio of oil to resin. The drying and curing time of all varnishes may be sped up by exposure to an energy source such as sunlight, ultraviolet light, or heat.
Many varnishes rely on organic solvents, or on organic oils or resins for their binder; these are highly flammable in their liquid state. All drying oils, certain alkyds, and many single-component polyurethanes produce heat during the curing process. Therefore, oil-soaked rags and paper can smolder or ignite hours after application if they are bunched or piled together, or, for example, placed in a container where the heat cannot dissipate.
Steel wool, also known as wire wool, is a bundle of strands of very fine soft steel filaments, used in finishing and repairing work to polish wood or metal objects, and for cleaning household cookware.
Steel wool is made from low-carbon steel (low enough to be close to plain iron). It is not made by drawing “steel wool wire” through a tapered die, but rather by a process more like broaching where a heavy steel wire is pulled through a toothed die that removes a thin wire shaving.
Rotten stone, sometimes spelled as one word, and also known as tripoli, is fine powdered rock used as a polishing abrasive in woodworking. It is usually weathered limestone mixed with diatomaceous, amorphous, or crystalline silica. It has similar applications to pumice, but it is generally sold as a finer powder and used for a more glossy polish after an initial treatment with coarser pumice powder.
It is usually mixed with oil, sometimes water, and rubbed on the surface of varnished or lacquered wood with a felt pad or cloth. Rotten stone is sometimes used to buff stains out of wood. Some polishing waxes contain powdered rotten stone in a paste substrate. For larger polishing jobs, rotten stone mixed with a binder is applied to polishing wheels.
It has also been used to polish brass, such as that found on military uniforms, as well as steel and other metals. Plates used in daguerreotypes were polished using rotten stone, the finest abrasive available at the time.
French polishing is a wood finishing technique that results in a very high gloss surface, with a deep colour and chatoyancy. Shellac is the purified secretion of the lac beetle, a sap-sucking insect from India and Thailand. It can be bought ready mixed, or in flakes or “buttons” and may be clear or tinted red (ruby) golden, (blonde) or other colors. French polishing consists of applying many thin coats of shellac dissolved in alcohol using a rubbing pad. The rubbing pad is made up of absorbent cotton or wool cloth wadding inside a square piece of fabric (usually soft cotton cloth) and is commonly referred to as a fad, (also called a tampon or muneca, Spanish for “rag doll”).
The pad is first used to put a thinned coat of shellac on, then thicker coats with small amounts of superfine pumice, a crushed volcanic glass. The pumice acts both as a fine abrasive and to fill the pores of open-grain woods. Each coat must be fully dry before the next application, to avoid lifting out the softened finish.
The finish is considered to be one of the most beautiful ways to finish highly figured wood, but it is also recognised to be fragile. It is softer than modern varnishes and lacquers and is particularly sensitive to spills of water or alcohol, which often produce white cloudy marks. However it is also simpler to repair than a damaged varnish finish, as patch repairs to French polish may be easily blended into an existing finish.
“French polish” is a process, not a material. The main material is shellac, although there are several other shellac-based finishes, not all of which class as French polishing.
The process is lengthy and very repetitive. The finish is obtained through a specific combination of different rubbing motions (generally circles and figure-eights), waiting for considerable time, building up layers of polish and then spiriting off any streaks left in the surface.
The ‘fad’ is commonly lubricated with an oil that is integrated into the overall finish. This helps to prevent the ‘fad’ from lifting previously applied layers of shellac. Typically, “softer” oils, such as mineral oil, will produce a glossier and less durable finish whereas “harder” oils, such as walnut oil and olive oil, will produce a more durable finish.
Additives to the shellac mixture include sandarac (sap of an African cedar) and copal, sap of a South American tree. These and other additives combined with heat and light can make the finish tougher, by cross-linking the polymers and oils in the shellac.
The piece is usually finished off after leveling (1500 grit oil sanding), then light buffing with carnauba paste wax. Too much heat or pressure from buffing will melt off the shellac and result in a bare spot that must be refinished.
French polishing became prominent in the 18th century. In the Victorian era, French polishing was commonly used on mahogany and other expensive woods, and was considered the best possible finish for fine furniture and string instruments such as pianos, lutes and guitars. The process was very labour intensive, however, and many major manufacturers abandoned the technique around 1930, preferring the cheaper and quicker techniques of spray finishing nitrocellulose lacquer and abrasive buffing. In Britain, instead of abrasive buffing, a fad of “pullover” is used in much the same way as traditional French polishing. This slightly melts the sprayed surface and has the effect of filling the grain and burnishing at the same time to leave a “French polished” look.
Another reason shellac fell from favour is its tendency to melt under low heat; for example, hot cups can leave marks on it. However, French polish is far more forgiving than any other finish in the sense that unlike lacquers, it can be easily repaired.
A brush is a tool with bristles, wire or other filaments, used for cleaning, grooming hair, make up, painting, surface finishing and for many other purposes. It generally consists of a handle or block to which the filaments are affixed, either parallel of perpendicular depending on the way the brush is to be gripped during use. The material of both the block and bristles, filaments, is also chosen to withstand hazards of its application, such as corrosive chemicals, heat or abrasion, this gives rise to a range of possible choices including plastic, metal, wood or even animal hair.