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.
- 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
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| 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.
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| 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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