Materials used today in buildings, plant and equipment include mainly mild steel, concrete, brickwork, aluminium, galvanised steel and timber. Steel and concrete are usually selected for areas subject to chemical exposure because of their inherent properties and, furthermore, they may be protected from corrosive action by the proper paint coating. Aluminium, galvanised steel and timber because of their characteristics are used in less demanding areas and they, too, are capable of protection by means of coatings. Irrespective of the material chosen, an adequate degree of surface preparation with due consideration of the environment is essential since it is on this basis that the adhesion of the correct painting system is dependent.
It is of supreme importance to consider the following factors in any decision of surface preparation:
Irrespective of the surface chosen, it is mandatory that before painting all dirt, dust, oil, grease or other loose surface contaminants be removed since it is obvious that paint applied to them will have poor adhesion to the underlying surface and the paint will flake off exposing the substrate to attack, resulting in costly maintenance, unsightly appearance and perhaps failure of the structure. Painting must always be carried out as soon as possible after and usually no later than the same day, as the surface preparation.
The following sections deal with each type of surface grouped under the following headings:
A detailed section covering surface preparation methods for steel is included.
Mild steel for its strength to weight ratio and cheapness is one of the most widely used construction materials. However, it readily rusts and must be painted to prevent this corrosion and to provide to it a decorative appearance.
Mill scale found on new steel is a hard, brittle coating of several distinct layers of iron oxides formed during processing of steel such as hot rolling girders, tank plates and other structural shapes. Usually bluish black in colour, mill scale cracks and fissures readily, and is permeable to both air and moisture. Rusting at the mill scale steel interface occurs and in time the scale sloughs off due to the pressure created by the rust layer. Mill scale is cathodic to the steel substrate and if left in place, corrosion will occur as a result of the electrical potential difference between them.
Rust is an oxide of iron formed by the action of air and water. It is voluminous and occupies one and three-quarter times the volume of the steel from which it originated. Rust forming under a paint coating or through breaks in the coating can burst through and may creep under the coating resulting in flaking so that repair is both difficult and costly.
It may cost a little more for a better surface preparation, but as the paint coating will last many times longer, the overall cost saving in maintenance will justify the initial expense. Other types of steel, such as low alloy steels (e.g. Austen 50), which are selected in areas requiring increased strength, hardness or improved resistance to corrosion, can also be prepared by the following methods.
The most commonly quoted reference standards for the preparation of steel for painting are given by the Steel Structures Painting Council (USA) or in the Swedish Standard SIS 055900.
A method of removing rust and mill scale by the physical impact of an abrasive propelled on to the surface by compressed air or by centrifugal force from a multiwheel machine. Non-metallic abrasives sand, ilmenite or copper slag are used for on site blasting, while in the fabricating shop a combination of round steel shot and steel grit may be used, the shot on impact breaks the mill scale and rust while the grit imparts profile or tooth to be abraded surface. If compressed air blasting is used, the air shall be free of detrimental amounts of water and oil. Adequate traps and separators shall be provided at the compressor.
The various methods of preparing steel for subsequent painting are given in the Steel Structures and Painting Council (SSPC) Standards of the U.S.A.
|Blast cleaning||SSPC||Swedish Standard SIS 05 59 00||Australian Standard AS1627- Part 4|
|Light or Brush||SP7||Sa1||Class 1|
|Medium or Commercial||SP6||Sa2||Class 2|
|Near White Metal||SP10||Sa2.5||Class 2.5|
|White Metal||SP5||Sa3||Class 3|
Types, sizes and resulting profile of abrasives used in air blast equipment
|Abrasive||Maximum particle size passing through mesh||Height of profile|
|Sand, very fine||80||1.5||40|
|Steelgrit # G-80||40||1.3-3.0||30-75|
|Iron grit # G-50||25||3.3||85|
|Iron grit # G-40||18||3.6||90|
|Iron grit # G-25||16||4.0||100|
|Iron grit # G-16||12||8.0||200|
|Steel shot # S-170||20||1.8-2.8||45-70|
|Iron shot # S-230||18||3.0||75|
|Iron shot # S-330||16||3.3||85|
|Iron shot # S-390||14||3.6||90|
Pickling (chemical descaling) is the removal of mill scale and rust using chemical solutions, usually acids, according to SSPC-SP8. Pickling cannot be used on erected structures and is essentially a process for the workshop. Carried out efficiently it is equivalent to white metal blast cleaning and leaves a relatively smooth surface on which it is often easier to obtain a more even paint coating. Surfaces descaled by chemical treatments are smoother than those produced by blast cleaning and may reduce the adhesion of some paints. With some very high build paint coatings the ‘tooth’ from a blast cleaning profile may be preferred.
A method of preparing steel surfaces by use of non-power hand tools. Hand tool cleaning removes all loose mill scale, loose rust, loose paint, and other loose detrimental foreign matter. It is not intended that adherent mill scale, rust, and paint be removed by this process. Mill scale, rust and paint are considered adherent if they cannot be removed by lifting with a dull putty knife.
Hand wire brushing, hand abrading, hand scraping or other similar non-impact methods are acceptable for the removal of loose mill scale, all loose or non-adherent rust and all loose paint.
Stratified rust (rust scale) and weld slag must be removed using impact hand tools.
Regardless of method use for cleaning, feather edges of remaining old paint so that the unpainted surface can have a reasonably smooth appearance.
Hand tool cleaning should only be specified when it is an acceptable method of preparation. It is only suitable for normal atmospheric exposures and interiors when the painting system includes a primer of good wetting ability.
A method of preparing steel surfaces by use of power assisted hand tools. Power tool cleaning removes all loose mill scale, loose rust, loose paint, and other loose detrimental foreign matter. It is not intended that adherent mill scale, rust and paint be removed by this process. Mill scale, rust and paint are considered adherent if they cannot be removed by lifting with a dull putty knife.
Power wire brushing, power abrading, power impact or other power rotary tools are acceptable means for removal of loose mill scale, all loose or non-adherent rust, and all loose paint. Do not burnish the surface.
Use rotary or impact power tools to remove stratified rust (rust scale) and weld slag.
Operate power tools in a manner that prevents the formation of burrs, sharp ridges and sharp cuts. Regardless of the method used, feather edges of remaining old paint so that the repainted surface can have a reasonably smooth appearance.
Metals included in the group comprise galvanised steel (also Zincanneal, Zincalume, and Galvabond, each of which are steel coatings with zinc and zinc and aluminium mixtures), aluminium and to a lesser extent copper, brass, bronze and zinc and aluminium metal spray.
The selection of galvanised iron and aluminium is increasingly used for roofing and cladding and both have a long history of satisfactory service when used for those purposes. When used in chemical or industrial plants or coastal atmospheric exposures a suitable paint system is necessary to give protection and added durability. The selection of a correct painting system is essential to prevent subsequent failure.
Adequate surface preparation is essential to ensure adhesion of the paint system and surface preparation with non-ferrous surfaces is usually confined to complete cleanliness such as removal of dirt and oil or grease, and this is adequately covered by Australian Standard CK 9.1, Degreasing of Metal Surfaces, which can be summarised as follows:
With brickwork and asbestos cement, the basic requirement is that the surface must be clean and dry. With concrete and masonry, additional problems may be caused by efflorescence, chalk and loose material, and from new or insufficiently aged concrete by:
One method of obtaining a satisfactory surface for painting is to blast clean carefully using a non-metallic abrasive such as sand or ilmenite, taking care not to expose aggregates unduly.
Alternatively the surface may be acid-etched. This treatment consists of treating the surface with dilute acids; either hydrochloric acid diluted with 6 parts, by volume of water, or phosphoric acid diluted with 10 parts, by volume of water. Wet the surface first with freshwater then apply the acid mixture, liberally, by brush or swab so that all areas show a bubbling reaction. After 5-15 minutes rinse off with copious quantities of freshwater and at the same time scrub with stiff brooms to remove loose concrete salts. Allow to dry, avoid any contamination of the etched surface and paint preferably within three days.
After this treatment the concrete should have a clean slightly roughened toothy surface.
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