Of dyes, resins and ‘French polish’

This is the second in a three-part series on wood coating and painting being published by WoodNews. To read the first part, check out the Sept-Oct 2018 issue of the magazine.

Chemical stains are derived from potassium permanganate, potassium dichromate, ammonia, lye, copper sulphates, ferrous sulphates and nitric acids.

By: DipakChaudhari

Staining is the process whereby wood surfaces can be made more valuable because their appearance is enhanced, and to bring definition to the wood grain.

All base stains, if used separately, are applied before the filler by spray or a pad and careful wiping helps to level out unequal absorption by the softer parts of the sapwood.

The main groups of base stains are solvent-based and water-based dye stains, gel stains and chemical stains. Dyes stains are those that actually dye/colour the fibres of wood. Stains such as aniline dye allow more “depth” to a finish and are often used for fine furniture. They are, however, photo-sensitive and fade with exposure to natural or artificial light.

Non-grain raising (NGR) dyes provide depth and clarity, have resistance to ultra-violet (UV) light and are, therefore, much more stable (lightfast) than aniline dyes. NGR stains are available in alcohol and acetone bases.

Dye stains are made from six base colours, and an infinite array of colours can be produced by mixing these. Adding solvent can reduce depth and intensity. Examples are Walnut, Rosewood, Oak Yellow, Red brown, Black, Green, Blue, Wenge, etc.

Highlights of dye stains:

•         No binder required as dye stains soaked by wood surface

•         Water and solvent soluble, impart brighter colour

•         Dyes are transparent colourants

•         Less resistance to fading due to light

•         Colour matching is easy.

Pigment-based stains deposit pigments between the fibres and into the pores of wood. They are stable and are most often recommended for architectural applications. Pigment stains are available in liquid or gel forms.

Generally the quality of stain application is judged by stability of the colour and uniformity of appearance. Inorganic pigments are derived from finely ground natural earth powder. They are dull in colour, have a dirty tone and are opaque, but are resistant to fading in sunlight.

Organic pigments are bright in colour, are manufactured by chemical reaction, can be opaque or transparent, but are less resistant to sunlight. Pigment stains require a binder and do not penetrate into wood. Pigment stains can lodge in pores, scratches and defects.

Chemical stains are derived from potassium permanganate, potassium dichromate, ammonia, lye, copper sulphates, ferrous sulphates and nitric acids. They bring about changes in the colour of wood by chemical reactions. However, it is difficult to control the intensity and depth of colours. The chemicals are difficult to use and often cause skin burn.

Exterior stains

These stains are used primarily on wood siding, decks and outdoor structures and furniture. They are available in latex and oil-based formulae. Oil-based semi-transparent stains offer the best protection of wood fully exposed to weather.

They can also be used on new or weathered wood, or wood previously finished with other penetrating finishes, without extensive surface preparation.

If a solid colour is desired, latex opaque stains are preferable over oil-based opaque stains, since they are more flexible, have better colour retention, and are less prone to mildew.

However, extensive surface preparation is required for proper adhesion, and latex stains in general should not be used on structures exposed to direct sunlight.

A clear or opaque wood coating, including clear lacquer sanding sealers, formulated with cellulosic or synthetic resins, if allowed to dry by evaporation, provide a solid, protective film.

Since this is intended as the final coat – in glossy or matt – it must have an excellent appearance and required aesthetics.

Top coats

There are many ways in which top coats can be used on wood-based furniture. Shellac is one of the few natural resins in use today, which is mixed with denatured spirit to form “French polish” or “Lacky”.

Shellac is not the best finish for tabletops, chairs or kitchen cabinets due to high wear of these items. But its big advantage lies in the fact that it dries very fast, making it unlikely to collect dust. It requires thinning with denatured alcohol before it can be applied with a brush or cotton rags.

Multiple coats of French polish give excellent look to furniture, but it is less durable and poor in water resistance, calling for regular maintenance of furniture.

For natural protection, oils and waxes were widely used. Their primary advantage is ease of direct application to wood and excellent finishing after buffing. Linseed and Tung oils, along with bees wax, are still preferred by some European manufacturers for special work pieces.

However, oil finishes cannot be built up to a thick coat like polyurethane or varnish. They offer less protection against wear and tear, heat and humidity. Oil coats dry slower, taking more time for recoating and increasing the chances of dust deposition on the wood surface.

Nitro-cellulose lacquers

Nitro-cellulose (NC) is a non-convertible polymer-based resin which forms fast-drying, hard, water- and heat-resistant and durable films. The film strength depends on the molecular weight of NC grade used. Two grades of NC are commonly used for lacquers: the ½-second NC and 30-40-second NC.

NC-based coatings are cured by evaporation of thinner. Thinner plays an important role in film formation of these coatings. NC is widely used in wood coatings as filler, sanding sealers and topcoat, applied by cotton rags, brushes or sprayed.

The main problem of NC is blushing – where the film turns milky if there is high moisture content. It is not soluble in solvents like acetates and ketones, but is compatible with resins such as short oil alkyds and ester gums. NC films are tough, but adhesion on smooth surfaces is poor.

It is also highly inflammable, to the extent of posing an explosion hazard, and must be stored away from electrical sparks. The use of NC has reduced due to development of synthetic resins like acrylics.

Cold curing

An acid cure (or cold cure) system is a special type of coating. It cures by the cross-linking of alkyd and amino resins with acid as a catalyst (hardener) to initiate the reaction. The high amount of cross-linking between the molecules is what makes acid cure system so durable.

The combination of toughness – its hardness and flexibility – and chemical resistance makes it ideal for kitchen and bath cabinets, office and institutional furniture, and other interior wood applications requiring durability.

When adding the acid to a base coating, it is very important to measure the acidic hardener correctly. Too little hardener will slow the curing rate and leave the coating system soft and less resistant to chemicals. Too much hardener will make it too brittle to withstand the expansion and contraction of wood.

But there is a rider: the acid cure system is susceptible to wrinkling of the finish. This usually occurs with the application and curing of the second coat of acid finish over an uncured acid cure base finish.

– The writer is a wood coating and finishing expert and consultant based in Pune. He can be contacted at woodfinishingconsultant@gmail.com.

To be continued…




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