CLEAR COATINGS

CLEAR COATINGS

 Post 119 - by Gautam Shah

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Clear coatings are primarily used to reveal the surface grain, colour and pattern, which however, are not always perfect or decent. Conditioning of the substrate surfaces is needed before application of a clear coating.

The most common problems with surfaces are like:

• uneven colour
• unsuitable tone
• uneven grain or pattern
• patchy absorbency
• uneven texture
• bleeding or soluble constituents
• waxy or oily deposits
• alkalinity
• acidity
• galvanic sensitivity
• moisture content and transfer

Shellac toning over pine wood stripe

Such problems with man-made surfaces are controllable, at industrial production level, but with natural surfaces like the Wood, have to be tackled on the site. To overcome the surface related problems in clear coatings, many types of surface treatments are used. Most of the surface conditioners or treatments are fast drying, easy to apply, non destructive for the substrate and insoluble in the subsequent coat material. Surface conditioners are of following types: Fillers (transparent or opaque), levellers, sealers, stainers (soluble and insoluble colourants), printed patterns, patterning tools, etching agents, bleaches, and reactants.

Fillers - Sealers are used to fill in pores so that coating material does not sink into it, and provide an even finish. Fillers generally consist of an extender, a binder and occasionally a colourant. Extenders are fine grade powder of materials like gypsum, chalk, china clay, precipitated calcium carbonate, lime, asbestine, colloidal silica, barytes and talc. Binders could be water, gums, oils, alkyds and poly vinyl emulsions. Solvent bound fillers are better compared with water bound fillers which raise the grain or fibres of the surface. Colourants provide the necessary tint, to white extenders. Fillers, made of pigment powders, serve the purpose of filling as well as staining. Transparent fillers (low viscosity NC lacquer, shellac, etc.) are used to fill the pores and provide a sealing coat to the decayable material in the grains and vessels filled with gum exhudents.

Stains provide a correct transparent tint to the surface. Stains are generally dye-materials, 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 exhudents. 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.

Clear coating on Leather

Bleaching process include 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. 

Imposed patterns: On surfaces where there are very irregular grains, patterns are screen-printed, pressed or embossed using stains. Such patterns may emulate a wood grain pattern, or are just very fine mesh or lines. Staining is also done by micro spray guns, singeing, burning or carbon deposition from flames.

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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