Post 182 –by Gautam Shah
Clear coatings are mainly used to show up the surface grain, colour and pattern. These however, are not always perfect or decent so need to be partially occluded or improvised. Conditioning of the substrate surfaces is needed before the 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
- galvanic sensitivity
- moisture content and transfer
Such problems with man-made surfaces are controllable at production level, but with natural surfaces like the wood, have to be tackled on the site.
Clear Finishes are also used where for avoidance of pigments with toxicity is required. Food packing, such as of paper, aluminium (cold drink tins), medical bottle stoppers, toys etc. require colourless coatings.
For clear coatings, to overcome the surface related problems many types of surface treatments are used. Most 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), levelers, sealers, strainers (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 body NC lacquer, shellac, etc.) are used, not only to fill the pores, but to 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-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 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.
- 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, and it also adds an amber hue to the coating on ageing.
- Imposed patterns: On surfaces where there are very irregular grains, the patterns are screen-printed, pressed or embossed using stains. Such patterns may emulate a wood grain or 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.