The decorator is seldom required to paint new radiators, which almost invariably are finished by the manufacturers, who are not only in a better position to know the conditions which the finish will have to withstand in service but are also able to treat the various sectional units separately before assembly and thus to ensure that a protective coating is applied to every part that needs it; this last point is especially important because, in cases where these appliances have to be treated in situ, the nature of their design makes some parts inaccessible or nearly so.
Although a properly formulated factory finish, applied under ideal conditions, should – and often does – last nearly as long as the apparatus itself, there are times when the decorator is called upon to repaint radiators, if only because a change of colouring is required. It is well to remember, therefore, that radiators cannot be regarded as belonging to the same category as ordinary ironwork, for, owing to the functions they fulfil as heating units, they demand certain qualities from the paint which other forms of ironwork do not require.
The points which need to be considered are that the finish shall not darken or discolour when subjected to considerable heat; that it shall be able to stand up well without becoming too brittle to maintain its adhesion; and that the paint coating applied shall interfere with the radiation of heat as little as possible.
The causes of the discoloration of paints under the action of heat can be attributed to faulty selection of driers and pigments. Lead and manganese driers, for example, exert a darkening influence on oil and varnish media in the continued presence of heat, and should be avoided or cut down to a minimum. Similarly, any pigment which gives rise to the formation of small percentages of either lead, iron, or manganese compounds of the oil should be avoided – at any rate, for the finishing coat. Zinc white, lithopone, and titanium do not discolour the oil on heating, nor does zinc oxide, but its tendency to form hard and brittle coatings is against it, and lithopone alone, or in conjunction with titanium, is to be preferred.
In view of the contraction and expansion which the film has to undergo, a high degree of elasticity is essential, and it is doubtful whether any paint which the decorator can make up for himself is capable of standing up to the conditions for more than a limited period. Some of the synthetic-resin type of enamels are excellent for this purpose, and special paints suitable for this type of work are made by most of the leading paint manufacturers. The best results are obtained by using primer, filler, undercoat, and finish made by the same firm and, in ordering them, it is well to advise the makers of these materials what is likely to be the maximum temperature to which the finish will be subjected.
As regards the effect of the finish on heat radiation, it must be said that this is a much-debated subject on which a number of contradictory views have been expressed. Probably the most authoritative opinion is that contained in a bulletin issued in December, 1944, by the Public Relation Department of the National Paint, Varnish and Lacquer Association of America. This states that the last coat of paint is the only one which has any appreciable effect on a radiator and that, under otherwise identical conditions, one finished with metallic paint (e.g. aluminium or bronze paints) will emit less heat than one finished with a non-metallic paint. Undercoats, it is stated, regardless of kind, have no significant effect on the performance of a radiator, except in the unusual case where the paint film is thick enough to act as an insulating covering. From the point of view of heat-release, therefore, it is unnecessary to remove the old finish when repainting a radiator. The result of adding a non-metallic paint as a finishing coat is, it is suggested, equivalent to adding one section to the radiator. Thus, a radiator of five sections, finished with a white or light-tinted paint, should be about as effective as another of six sections of the same land, finished with metallic paint.
In repainting radiators, the first step will be to determine whether the existing coating is in sufficiently good condition to act as a ground for the new finish, or whether it is necessary to strip it. In cases where repainting has already been carried out, it will often be found that the old paint has become fairly brittle, and will thus tend to shell off without much difficulty.
Small wire brushes and coarse sandpaper will be found helpful. All joints and flanges should then be carefully wire-brushed and the radiator tested for leaks, which should be remedied before painting is commenced. The surface should be cleaned with turpentine and any blow-holes or defects in the casting made good with paste filler.
An effective system of painting radiators is to apply each coat of paint thinly, with the radiator cold, using slow-drying paints with very little driers, and then, when each coat has become dry to the touch, to turn on the heat for a few hours – moderately at first, gradually arriving at full heat and then cooling right down before the next coat is given. This treatment tends to harden the paint.
It may, of course, not be possible for this method to be followed, but it is usually worth while trying to make arrangements for it to be carried out.