Linseed oil alone will give moderate heat and water resistance with high gloss if applied sparingly over a period of several months. Each separate application must harden by oxidization, and 5 per cent white spirits and 5 per cent terebene can be added for easier working, but at least 12 coats will be necessary. This method was often used for dining-table tops but it is much too slow for modern requirements, and sophisticated penetrating oils, teak oil etc., have been developed, incorporating rapid oxidizing agents capable of achieving a sufficient build-up in two coats only. These new oils are used extensively in modern teak and afrormosia furniture, but the resultant finish hardly compares with the older process. As with wax, oil finishes are easily renewed, which is one of the chief advantages.
This is a specialist craft and should not be attempted without proper training, for the whole secret lies in the minimum and not the maximum amount of polish applied for a full gloss, and thick coats unskilfully applied are no better than cheap varnish. It is often said that a good french-polisher is born and not made, and that not only can he equalize the colours of mixed woods but can actually make some pieces appear lighter by regulating the thickness of polish and therefore the refractive index. Commercial brands of polish incorporate various gums and synthetics for quick drying and build up. also increased heat and water resistance which is usually poor with straight french polish, but the most beautiful finishes are still obtained with the older method, using flake-orange or garnet shellac dissolved in industrial alcohol (methylated spirits, methylated finish, etc.). The best results are only obtained over long periods and 12 months
546 Applying rubber in french polishing a table top
was considered usual for the German vitriol finish on grand pianos. The polish as such is now little used in production work for it is essentially a hand process, but modified polishes are available for spray finishing.
These are composed of nitro-cellulose, alkyd resin and castor oil or glycerine plasticizers, etc. dissolved in various aromatic hydrocarbon solvents according to the type. Originally developed some 40 years ago to replace french polish they have good heat-, water- and spirit-alcohol-resistant properties, and in spite of fierce competition from harder and more resistant synthetic lacquers, still account for about 70 per cent of the total output, presumably because they are familiar, can be used straight from the can and have an unlimited shelf and pot life. Obvious advantages are a very quick drying rate giving dustproof coats in a matter of minutes, high transparency, an exceptional even flow and good gloss together with satisfactory amalgamation of successive layers, as with french polish. This last factor is of considerable importance, for each coat applied partially redissolves the underlying coat, and therefore any subsequent damage is more easily repairable than with synthetic lacquers, which set by polymerization so that the separate layers do not amalgamate and any renewal usually necessitates stripping down to the bare wood.
Normally, all cellulose lacquers are compounded for spray application and have a solids content of around 34 per cent with low viscosity and a flash point of 32° C (90° F); they are therefore subject to the requirements of a number of workshop regulations. However, full gloss lacquers can be obtained with a flash point of over 32° C (90° F) which are outside the regulations, but owing to the slower solvent evaporation-rate they do not have the rapid setup of normal lacquers. They can, however, be brush coated, which is satisfactory for thin coats but rather more difficult for a filled grain full gloss over large areas. Cellulose lacquer can be 'pulled over' as in french polishing, with a chamois-leather pad soaked in pull-over solutions composed of high-boiling retarder thinners; recent developments include straight cellulose lacquers for pad polishing, high solids content lacquers for quick build-up. and scratch-resistant lacquers with resistance to bruising without fracture, which was one of the disabilities of the older, more brittle formulas.
For those who do not have spraying facilities and prefer soft shine with reasonably good protection, a proved specification is as follows:
One coat or more well-thinried clear cellulose lacquer, brushed on, allowed to harden thoroughly, and then wire wooled (grade No. 0000) and waxed, preferably with the lacquer-maker's own matt wax formula.
This treatment yields an acceptable finish for the general run of handmade furniture, but will not give full protection on dining- and coffee-table tops. Either white french polish or polyurethane lacquer can be used for the base coats in lieu of cellulose; but whichever lacquer is used some judgment is needed, for if the coat is too thin rubbing down will strike through to the bare wood in places and show as bald patches under the wax, while too thick coats will merely lie on the surface and show the brush-marks.
These include cold acid catalyst, polyurethane and polyester lacquers, all of which have greatly increased heat, water and spirit resistance, tough films and high gloss, particularly the latter which is extensively used for wireless and television cabinets as there is little if any sinkage, and the gloss is comparable with a vitriol (German) piano finish. Polyester lacquers are, however, unsuitable for small turnovers, for they require dual-feed sprays for intimate dispersal of the catalyst hardener throughout the lacquer, and the resin-dust in surfacing can be an industrial hazard if suitable precautions are not taken.
Originally phenolic resins were used, but modern formulas include urea formaldehyde, melamine and epoxy resins with alkyd plasticizers for increased resistance to wet, heat, etc. A two-pack lacquer is the norm,
547 Portable rotary vane air compressor. (By courtesy of A. Bullows and Co. Ltd)
composed of lacquer, syrup and separate acid catalyst, often hydrochloric acid, which must be mixed with the lacquer in the stated proportions. Setting is by polymerization, but the mixed lacquer has a pot life of several hours and enough can be mixed for the day's work. A one-solution pre-catalysed form is now available with a shelf life of about six months, but the pre-catalysation is obtained by lowering the solids content, and therefore the build-up is less rapid.
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