The chief aim in painting iron and steelwork is to protect the metal from corrosion and, if this objective is to be attained, great care must be given to the preparation of the surface as well as to the choice of paints.
New iron and steelwork has a coating of mill-scale; if this is intact, it provides a good foundation for paint and, on occasion, the latter is applied at the factory while the metal is still hot after fabrication; as a rule, excellent results are obtained by this process. To paint scale-covered surfaces on the site, however, is a hazardous procedure because the scale nearly always contains flaws and cracks which may contain minute areas of rust and this may subsequently loosen the scale and attack the paint. Weathering for a period of several months will detach the scale, but unless all of it is removed before painting, there is great risk of premature failure of the paint, since the scale will break away, bringing the paint coating with it.
For this reason, before structural steel is transferred from the works to a building site, or before it is painted, it is fairly common practice to give it a chemical pre-treatment or to remove the scale by pickling or sandblasting. Chemical pre-treatments, such as the Bonderising or Parkerising processes, deposit a phosphatic layer on the metal and this provides a reliable protection against corrosion as well as a good key for paint; such processes, however, are only suitable for relatively small surfaces. Pickling is done by means of acid solutions, the chief objection being the risk that traces of acid or water may be left on the surface to the detriment of the paint film. Sand-blasting (or shot-blasting) imparts a key to the metal which greatly assists the adhesion of the paint film, but this treatment is not always successful in removing scale which adheres tightly or which is in crevices or less accessible parts of the work.
During the last war a process known as ‘ flame-priming,’ which had been introduced previously, was greatly developed on a large scale in the U.S.A. shipyards. It involves the use of a standard welding blowpipe fitted with a special head through which passes a series of closely spaced oxy-acetylene flames with an extremely high temperature and velocity. These are played upon the surface of the metal with the result that all mill-scale which is not firmly bonded is loosened by the sudden thermal expansion, while all chemically combined water is driven from any rust which may be present, leaving a warm, dry, granulated powder which is easily removed by brushing. Other contaminants, such as oil or acids are consumed or disintegrated. The surface is then wiped free from all foreign matter and painted while still at an elevated temperature, the heat of the metal promoting a freer flow of the paint. This process has found its way to this country and, as the equipment involved is not unduly complicated, will probably be increasingly employed in the near future, since it obviously has great possibilities.
Iron and steelwork which has been descaled soon corrodes if it is unpro-tected; because of this, a ‘ shop ‘ primer is not infrequently applied at the works. There is an obvious advantage in this provided that high-grade paint is used and put on by skilled painters. Too often, however, cheap and inferior paint is brushed on roughly by semi-skilled labour and has little preservative value. Indeed, it may even be necessary for the painters on the site to remove it and replace it with a primer of good quality.
Needless to say, rust will give more trouble in exterior work on such surfaces as those of gutterings, down-pipes, iron window frames, railings, etc. Prevention is always better than cure, and when these various surfaces are new or have been stripped, prior to repainting, they should be examined with the utmost care for any signs of corrosion.
It is, however, a difficult and usually an expensive business to remove rust, but when it is considered that no amount of painting over it will check its spread, once it has begun, it is obviously useless to waste paint and labour on ironwork which shows any trace of rust or scale.
Wire brushes are generally employed, and are extremely useful for this purpose. They are not always effective, however, since they have a tendency to remove only the loose deposits and to polish over the surface of rust which is more firmly attached. In bad cases, scraping with old sharpened files, blunt chisels, or specially shaped knives is usually more thorough.
Assuming that the ironwork is free from rust, the cleanliness of the sur- face is the next consideration. Here, the greatest danger is undoubtedly from grease, which, when picked up on a smooth surface such as metal, is liable to spread; even a relatively small amount, which would probably sink into a more porous surface and give very little trouble, is quite sufficient to prevent proper adhesion of the paint to the metal. It is as well, therefore, to make it a rule to cleanse all iron or other metal surfaces with white spirit or benzine before any painting is attempted.
The chief difficulties in painting metal surfaces are due to their smooth, non-absorbent surface, their comparatively large expansion and contraction under extremes of heat and cold, and their great heat conductivity. The lack of porosity makes it more difficult for paint to gain and maintain an anchorage, and this is accentuated by the expansion of the metal in the direct heat of the sun or from other normal sources.
The absence of suction is not without its advantages, however, for provided adequate adhesion is obtained, it enables the ratio of pigments to vehicle to remain the same after the paint has been applied and has set, whereas, with porous surfaces, the ratio may be completely upset.
Assuming that the surface is free from rust, dirt, and grease, priming should proceed with a minimum of delay. It is highly important that the surface should be quite dry, and for this reason painting should not be carried out, if it can be avoided, in humid weather or too soon after dew has fallen; in such conditions a certain amount of water may have been deposited on the surface, though it may be almost invisible to the naked eye. Even a minute quantity of water is capable of promoting corrosion and leading to a premature failure of the paint.
In applying the paint, special attention should be given to rivet and bolt heads, nuts, and other protuberances, and the brush should be well worked into all corners and angles. Care should be taken, too, that all edges are properly coated; with inexperienced or careless painters the tendency is either to leave ‘ fat ‘ edges or to allow the action of the brush to remove too much paint on such areas.
Choice of Paints
Many different types of paint are, at times, recommended or specified for use in priming iron and steelwork, and it will be helpful to summarise the various advantages and disadvantages of some of those commonly employed, classifying each paint according to the pigment on which it is based.
The standard anti-corrosive pigment, and without doubt the most extensively used for the purpose, is red lead which, by reason of a mild alkalinity, possesses the ability to counteract the acid conditions which are recognised as promoting corrosion and to continue to neutralise them for a prolonged period. Its rust-inhibitive properties are, however, fully developed only when the paint is in direct contact with the bare metal and are not exerted if, for instance, a red-lead paint is applied on top of a shop primer formulated with another type of pigment. Red lead has a powerful drying influence on linseed oil, thus making the inclusion of additional drying agents unnecessary. It has great adhesion but does not spread easily under the brush, though its working properties can be improved by mixing it with a proportion (about 25 per cent.) of extender pigment; this does not substantially decrease the anti-corrosive value of the paint. The rapidity with which it settles in the container is a serious objection to the use of the ordinary variety of red lead; the non-setting variety is, of course, free from this defect but has not quite the same high degree of rust-inhibition. A red-lead primer provides a good foundation for further coats of paint but needs the latter to give it protection from abrasion and wear and tear.
Red Iron Oxide
Red oxide of iron gives good service as a primer for ferrous metals; it is relatively cheap, covers well, and is widely employed for the purpose. The pigment, if pure, has no special anti-corrosive properties and is probably most valuable, in this connection, when used in conjunction with an inhibitive pigment, such as red lead.
Lead and Zinc Chromes
Primers based on the chromate pigments are expensive and this, no doubt, limits their use, for they have excellent anti-corrosive properties and are more easy to apply than is red lead. As a rule, they are employed not by themselves but mixed with a cheaper pigment, such as red iron oxide, and give good protection against rust.
At one time graphite found a good deal of favour as the base of a priming paint for iron and steelwork. Though it has good moisture resistance, due to the structure of its particles, it is now generally realised that it is unsuitable for use on bare ferrous metals since, far from being rust-inhibitive, it actually tends to promote corrosion. For undercoats and finishing coats, however, there is much to be said for its employment.
Aluminium powder, mixed with a suitable vehicle, provides a paint which has excellent adhesion and, owing to the leafing properties of the metal, exceptional powers of moisture resistance. It is not rust-inhibitive and, like graphite, is most useful when applied over a good anti-corrosive primer.
Provided the right type of vehicle is selected, zinc dust gives a primer which affords good protection against corrosion, though it is not used on an extensive scale for this purpose. Recent tests have shown it to be an excellent pigment for priming new galvanised iron, and mixtures of zinc dust and zinc oxide have yielded good results.
Only a comparatively limited use is made of metallic-lead pigments for priming paints for iron and steelwork but, generally speaking, they give good results, their value, in this connection, being due to their durability and moisture resistance. f
Of the vehicles used for primers for iron and steelwork, linseed oil is the most popular. Raw linseed oil has relatively poor properties of moisture resistance, and heat-treated oils, such as stand oil, are superior in this respect. Tung oil is still better but, owing to its cost, its use for this purpose is restricted. Suitable gums, resins, and waxes are often added to linseed oil to make it less permeable to moisture, and some of the newer synthetic resins are especially valuable for the purpose.
Bituminous Paints and Compositions
Bituminous paints, made from asphaltum or pitch and reduced to working consistency by means of a suitable solvent such as coal-tar naphtha, are often employed for protecting iron and steelwork and are exceedingly effective for the purpose; with certain types, thinning is accomplished by heating.
These compositions are unaffected by acids or alkalis and give coatings with great resistance to moisture penetration. Unlike most other forms of paint or varnish, they are most effective when applied in thick coats. The film retains a certain amount of plasticity indefinitely and never becomes really hard. When exposed to strong sunlight for any length of time, these coatings tend to disintegrate fairly rapidly, but this can be prevented to a large extent by incorporating a proportion of aluminium powder in the mixture. From the painter’s point of view, a formidable objection to the use of bituminous paints is that they will bleed through and discolour subsequent coats of paint applied directly on top of them; it is quite true that by interposing a sealing coat of shellac this defect can be prevented, but the application of a hard and brittle film of shellac over a soft and elastic bituminous coating is liable to cause cracking of the finish and cannot be considered sound practice.
In discussing the priming of iron and steelwork, mention should be made of the process usually described as ‘ cork-dusting ‘ and frequently carried out in the engine-rooms of ships or in extra-humid situations, such as laundries, to reduce the risk of condensation.
The chilling properties of metals are familiar to most people and when warm, moist air comes in contact with a metal surface it will nearly always happen that moisture will be deposited upon the latter. Water pipes present this difficulty in exaggerated form because they form a water-cooled device for the condensation of atmospheric moisture ; even gas pipes near the ground may give some trouble at times.
In cork-dusting, the ironwork, after having been cleaned and the rust removed from it, is coated without delay with a stiff paint or varnish, and while the latter is still tacky, cork dust is thrown upon it, preferably by means of special bellows. After the paint or varnish has hardened, any loosely adhering cork particles are dusted off with a soft broom and painting is then completed in the ordinary manner. The finished surface is thus separated from the metal by a thin but efficient layer of non-conductive material, and far less condensation of water takes place. The presence of the cork particles gives the coating a granular appearance which is not very attractive, but the process, in most cases, is carried out for a purely utilitarian purpose.
Priming Galvanised Iron
A great deal of galvanised iron is left unpainted, since it is considered to be rust-proof. This belief is justified, but only up to a point. As long as the coating given to the metal in the galvanising process remains intact, there will be no fear of corrosion, but it should be remembered that galvanising is carried out on large sheets which are subsequently cut into various shapes and sizes, drilled for screws or rivets, and frequently badly scratched in handling or assembly; ‘ raw ‘ edges and areas are thus exposed and, on the great majority of galvanised iron which has stood for any length of time without having been painted, traces of rust can be found and these are liable to spread. So, quite apart from the fact that the unpainted metal has a somewhat unsightly appearance, there is a good case, from the preservative point of view, for painting it.
New galvanised iron cannot, however, be treated in the same way as ordinary iron or steelwork, because of the difficulty of getting the paint to adhere properly to the zinc coating. It is necessary to impart a key to the. latter. This can be done by allowing the sheet to stand exposed to the weather for about six months so that the zinc can become lightly oxidised. This method is open to the objection that the zinc coating is liable to become pinholed so that rusting may begin, and may be all the more dangerous in that the areas thus affected may be so small as to escape notice.
The treatment more commonly adopted is artificially to etch the surface of the new metal by means of various chemicals in solution. Most manufacturers supply proprietary preparations for the purpose but many painters prefer to make up their own. If they do so, they should remember that most of the ingredients recommended may have a powerful action and must therefore be used with restraint. For example, muriatic or hydrochloric-acid solutions are effective in this connection, but only very weak solutions must be employed and they must not be allowed to remain in contact with the surface for too long or the zinc coating will be dissolved in spots and the iron base will be attacked. Their application must be followed by thorough rinsing off with plenty of clean water.
A reliable form of treatment is to use a solution of copper sulphate in the proportion of about 5 oz. to a gallon of water. This should be applied fairly liberally, left to dry, the surface carefully washed with cold water to remove the black deposit, leathered off, and allowed to dry. The first coat of paint, which should preferably be a red lead or other good rust-inhibitive paint, should be applied without delay.
Alternative materials which can be used to etch the surface are a weak solution of acetic acid (vinegar) or one of soda (1 lb. to 5 gal. of water), though neither of these is, in practice, so effective as the copper-sulphate treatment. Before any solution is applied, care should be taken that the surface is free from dirt, grease, or other superficial impurities.
Once a key has been imparted to the surface, the metal can be regarded, for the purpose of painting, as ordinary iron and primed and finished as already indicated.
Other metals are seldom required to be painted, but in cases where a paint finish is wanted, the following notes should be helpful.
The surface of the metal must be scrupulously clean. If copper has been allowed to stand exposed to the air for some time, a deposit of verdigris will have formed; this can be removed by means of a rag moistened with ammonia, remembering that verdigris is poisonous, so that the rag should be handled with care and burnt after use.
Unless the surface of the metal is roughened some difficulty will be experienced in making the priming coat of paint ‘ key ‘ properly. As a rule, rubbing with fine emery cloth will etch the copper or bronze sufficiently for this purpose and this is preferable to an alternative method sometimes adopted – the application of a weak solution of nitric acid; if the latter is used, the copper or bronze must be well rinsed with clean water and dried with a leather before the paint is put on.
Since the metal presents an impervious surface, the first coat must be of a naturally tenacious nature. A thin coat of gold size is quite suitable, and this can be followed by one coat of undercoat, and one of finish, in the usual way. If a better finish is wanted, a second coat of undercoat, consisting of a mixture of equal parts of the paint used for the first undercoat and of the finishing paint or enamel, can be given before the final coat of finish. Flat down between coats and take care that only thin, even coatings are given.
As for copper and bronze.
Although slight surface-roughening by means of a fine abrasive is advocated by some authorities, others consider it unnecessary and even undesirable, as it may affect the natural oxide film always present on aluminium. Cleaning with acid or alkaline solutions should be avoided and a solvent such as turpentine or naphtha used for this purpose instead.
As regards the choice of paint for a primer, lead should not be used, either in the form of white-lead or metallic-lead pigment, or in driers added to the paint. A zinc-chromate primer gives good results or, if the conditions to which the work will be exposed are not too severe, a good alumin- ium primer with a bodied oil or elastic varnish medium. Follow on with undercoats and finish in the usual way.
The chief difficulty in painting lead arises from the film of oxide on the surface which prevents good adhesion. The lead should be rubbed down with abrasive until the metal shows bright and the primer applied while it is still in this condition. The first coat may be either a good-grade gold size or a white-lead primer, in which a proportion of the vehicle is a good-grade mixing varnish. Once the primer is hard and dry, the painting can be completed in the ordinary manner.