Any steel upon which it is de sired to apply a preservative coating should have its surface in such condition also that the coating will adhere well to it. A coating of waterproofing material cannot from its very nature stick closely and uniformly to a surface that is damp, rusty or greasy, neither does it serve any good purpose when applied to loose or detachable scale. Engineers differ as to the best method of securing a receptive surface for painting; however, much of the expense of re painting in the future would be saved if all scale, rust, grease and dirt were removed from the surface of the metal at the steel-making shops before applying the first coating of paint to new work. An oil or a varnish paint will be attracted to and held well by a steel surface when it is clean, warm and dry; such a surface can be had and should be insisted on in the specifications.
All steel as it leaves the rolls or hammer has a tough, thick or thin coat of loose or partly loose scale that adheres for the time being, buf on short exposure to the air with a few changes in temperature, due to mill or storehouse conditions, releases its tension and is liable to fall off whenever handled, as in course of loading and transportation.
Steel that is subjected to machine operations in assembling shops or places where bridges are fabricated is more or less covered with machine grease.
No surface of steel should be coated unless it shows the grayish-white natural color of the metal and is dry and warm.
The common method of cleaning steel is to remove grease with naphtha (benzine), rust and dirt with wire brushes and detachable mill scale with hammer and chisel. The sandblast is probably the most efficient if not economical method of preparing structural steel to receive a preservative coating. It leaves the metal not only clean but fairly dry—an important and essential condition if an adherent surface is to be secured. The painting of the metal should follow immediately after the cleaning is done, as cleaned metal will corrode under ordinary atmospheric conditions much more rapidly than uncleaned metal.
Portable sand-blast machines and portable air compressors are now available. The out fit besides these includes a sand drying ap paratus, hose, nozzles, etc. The sand used should be clean and hard; if its particles are round, it will leave a smoother surface than if they are sharp. Where the surface is very rusty and the metal pitted, a coarser sand is needed than for the removal of ordinary weather rust. It is argued against sand-blast
ing that it should not be used to clean new metal because it will remove the sub-oxide, black oxide or mill scale, which in itself, like paint, is a protection against rust formation so long as it remains. While this is true if the scale is entire, it does not hold where it is in jured; and is without the certainty of the sand-blast method.
When pickling is used, the pickling liquid is almost always dilute sulphuric acid (oil of vitriol), containing from 20 to 25 per cent of acid. It is preferably used warm, and the articles are left in it till the surface is per fectly clean and free from rust and scale. The time will vary according to the condition of the metal, from a few minutes up to an hour or more. A much weaker acid than that mentioned. is sometimes used, or the acid is used cold, but in these cases the article must be cleaned with alkali before it is put into the pickling vat. After the pickling, the acid must be removed from the surface as rapidly as possible, and this j is best done by means of a jet of water dis charged at high pressure, mere rinsing not be ing efficient for the purpose.
Hydrochloric acid (muriatic acid) is not so satisfactory as sulphuric acid. If the metal be allowed to dry in the air the surface is likely to rust. To avoid this it should be put into a boiling bath of milk of lime, and left in this until it has acquired the temperature of the bath. It is then taken out, dried in an oven and when dry this lime is brushed off. The surface left by pickling is susceptible to rust, and, therefore, a protecting layer of some kind must be applied at once.
With present facilities at steel plants and assembling shops, neither space nor time is available for cleaning steel, either by pickling or with the sand-blast, so the common practice is to substitute quicker and less reliable methods of preparing the surface to receive paint. The process of galvanizing is described under that title. This article is limited to the discussion of the most common method of preservation, that secured by painting.
At the present time oil paints are generally used for coating structural steel. While varnish paints may be more efficient for metal not exposed to the action of rainfall and sunshine, or under some special conditions, experience indicates that oil paints are the proper coatings for preserving and protecting steel used in building and engineering con struction.