WOOD SURFACE PREPARATIONS for CLEAR COATINGS

Post -by Gautam Shah

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Chinese Style Redwood Wood Bathroom Cabinet

Natural Woods are many different types, such as Soft, Medium and Hard grain. Woods are likely to be Fresh, Seasoned and Old woods. Woods have different types of faces such as the end grains, flat and side faces. The woods have problems of uneven colours, patterns. The variations in textures are both natural and tools or machine made.

Soft Woods Pinus sylvestris

Wood products have similar problems but an average quality due to the mass production processes. The variations however are between near-natural products (plywoods, veneers) and synthesized products (particle boards, MDFs etc.)

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For a clear coating application all wood surfaces require surface preparation treatments and post coating application treatments. The surface preparation treatments are over and above the nominal craft processes such as planing, sand papering, etc. These are mainly Filling, Sealing and Staining. The post coating application treatments are not always required, but could be Buffing, burnishing, waxing, etc.

Hardwood tops

Fillers

Fillers generally consist of an Extender, a Binder and occasionally a Colourant. Fillers are required to fill in the pores in the wood grains of Natural woods and cavities in case of wood products. A levelled surface provides better gloss and integrity of the coating (one continuous surface without breaks). The coating material does not sink into it, and provide an even finish. Transparent fillers (low body NC lacquer, shellac, etc.) are used not only to fill the pores, but provide a sealing coat to the decayable material in the grains and vessels filled with gum exudates.

Extenders

Extenders are low opacity -reflectivity, fine grade powder of materials like gypsum, chalk, china clay, precipitated calcium carbonate, lime, asbestine, colloidal silica, barytes and talc(unlike the Zinc oxide or Titanium dioxide -the white pigments that adds whiteness and opacity).

Binders

Binders are binding agents that hold together the extenders, temporarily or permanently, and also bind the extenders to the wood substrate. A binder could be water, gums, oils, alkyds and poly vinyl emulsions. Solvent bound binders are better compared to water bound binders. The later ones raise the grain or fibres off the surface.

Colourants

Colourants provide an equalizing tint to the wood surface and slightly colour the white extenders. Fillers, made of low opacity-reflectivity pigments serve a dual purpose, of filling as well as staining.

Wood Stain Brush Delete Planks Wood Wax Wood

Stains as colourants

Stains provide a correct transparent tinge to the surface. Stains are generally dye-material soluble in water, oil or solvents. Water soluble stains though of many different varieties, raise the fibres and are difficult to penetrate. Oil soluble stains are heavy bodied, take longer to dry out and interfere in the subsequent coating application. Solvent soluble stains are costly, dry out immediately and may bleed residual gums and other exudates.

Wood Staining

Stains have one important drawback that they darken the existing colour of the substrate. Where timber surfaces need to be of lighter colour, surfaces have to be bleached or toned with opaque materials.

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Bleaching

Bleaching process lightens the existing colour of the wood. It includes a treatment with hydrogen peroxide followed by an alkaline accelerator like lime, caustic soda, sodium silicate or ammonia. Bleaching affects adhesion and toughness of coating. It also provides an amber hue to the coating on aging. Staining is also done by micro spray guns, singeing, burning or carbon deposition from flames.

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Imposed patterns

On wood surfaces where there are very irregular grains or no patterns, these are screen-printed, pressed or embossed using stains. Such patterns may emulate a wood grain pattern or just very fine mesh or lines.

Wenge veneer imposed pattern

Post application treatments

Clear finishes often require some post application treatments. These are mainly burnishing and waxing. Burnishing is mainly done to NC lacquer, Acrylic and Melamine coatings, to provide a glossy surface. Burnishing is not done to slow drying coatings, because such coatings, though are dry on outer face, take days to thoroughly dry out. Burnishing is done with a Carborundum like rubbing material with a waxy or oily base. Waxing provides a dull sheen and a protective coating. Waxing is also done to renovate old coatings. Waxing compounds also include a small amount of oils and sometime silicone materials.

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LACQUERS or NC LACQUERS

Postby Gautam Shah

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Lacquer is a very generic term applied to a variety of coating systems. It indicates a surfacing system, where a film forming substance has been dissolved in or diluted with a solvent. On application the solvent evaporates leaving a thin film on the surface.

The lacquer, on evaporation of its solvents changes the phase from Liquid to Solid, but does not convert itself into different substance. Lacquer films are soluble in the (original) solvent. Lacquers are also thermoplastic (will soften with an increase in heat) materials. The film forming substances of lacquers are linear polymers high in polymer weight with good solvent releasing property. Such substances are often very hard and brittle, so require addition of plasticizers to achieve better adhesion, flexibility and durability of the gloss.

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Commercially the term Lacquer is applied to Solvent-based Coating systems (against water-based systems) that offer hard, glossy and durable film. In this sense, there are Two basic varieties:

1 Thermoplastics type: Nitro cellulose, Acrylics, Chlorinated Rubber, Vinyls, Epoxy.

2 Thermosetting type: Acrylics, Epoxy Resins, Polyurethanes, Vinyls, Alkyd, Melamine.

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Cellulosic Polymers

Cellulose is the most widely occurring, a natural polymer, available in almost all plants. Primary sources of industrial cellulose are cotton fibres and wood. Cellulose is a polyhydric-alcohol, and forms esters with organic acids. It also forms reaction products with inorganic acids. These cellulosic derivatives used in production of various types of lacquers are classified as:

1. esters of inorganic acids -nitro cellulose

2. esters of organic acids -cellulose acetate

3. others -methyl and ethyl cellulose

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Lacquers can be classified into two basic groups:

NC Lacquers and Acrylic or Plastic Lacquers.

Lamellar Lines Mudguard Horizontal Lacquered Sheet

Nitro Cellulose Lacquers

NC Lacquers consist of Nitro cellulose as the main raw material to which a variety of plasticizer and resins are added, to provide wide range of film properties. PLASTICIZERS increase elasticity and extensibility of the film. Commonly used Plasticizers are, blown-castor oil, dibutyl phthalate, dioctyl phthalate, dibutyl sebacate, butyl stearate, chlorinated diphenyls. Additions of RESINS increase gloss and adhesion of the film. The resins are coconut and castor oil alkyds, maleic modified ester gum, cyclohexanone (ketonic) resins, acrylic resins, toluene sulfonamides formaldehyde resins.

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NC Lacquers are produced by dissolving or diluting lacquer and suitable plasticizers in a mix of solvents. The SOLVENTS act in three ways: Active solvents are (ethyl acetate, butyl acetate), Latent solvents (ethyl alcohol, butyl alcohol, diacetone alcohol), and Dilutents (toluene, xylene). A suitable mix of these basic three types of solvents is known as `Thinner‘. A very specific quality of Thinner is required for each application and weather condition.

NC Lacquers do not become yellow with ageing. It has no peeling problem. Lacquers provide very ‘water-white’ (no colouration or tinge) film. NC Lacquers are re-coatable and removable substances.

NC Lacquers are used on wood, metals, metals like gold, silver and copper, plastics, leather, paper, fabrics, as hair-fixers, and Nail-polishes. Lacquer is one of the best surface sealing material for wood products and so forms the primer coat for melamine, etc. 

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Solvent-based nitrocellulose lacquers (from resin of nitration of cotton (lint) and other cellulose based materials, were used commercially from the 19th C. These lacquers began to replace shellac lacquer and were used as protective or rust inhibitive clear coatings for brass musical instruments, gift items and furniture. These were also used in white-goods products (refrigerators, irons, cooking tops, etc. and in automobiles. These lasted for several decades till arrival of ‘Polymer based lacquers‘.

These materials are called Lacquers because the film is ‘water-white’. Acrylic lacquers of thermoplastic type are characterized by a transparent water white film, very good resistance to decolouration from temperature and UV light, good electrical properties, outdoor durability, excellent resistance to mineral oil-greases, resistance to chemical fumes, resistance to water based alkaline substances.

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Acrylic lacquers convertible coating that is on drying covert to a different product, which is non-dissoluble and non-removable. These are used for: wood items, clear coating for brass, copper, and aluminium, as a top coat for final gloss and protection air-crafts, automobiles, toys, auto-parts, industrial components, consumer durables, TV cabinets. These are used as clear-invisible coating for exposed surfaces like bricks, stones and concrete. Acrylic lacquers with soluble dyes form transparent glass coatings. One of the major uses is for internal coatings of food and beverage cans.

Thermosetting Acrylics provide a hard and scratch proof coating, good colour retention, better water and detergent resistance, better gloss and chalking resistance.

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