Variety of coating materials are applied to a surface to protect it against corrosion, erosion and rebuild damaged components. Next to proper selection of a coating material, surface preparation is the most important procedure. Better surface preparation results in a longer life. Surface preparation methods, surface preparation standards, application methods and safety precautions during coating application are covered in this article.
Surface Preparation
Detail on surface preparation methods for concrete / masonry, non ferrous metals and steel is as under.
Surface Preparation of Concrete and Masonry Surfaces
Surface should be clean, dry, free from oil/grease and other contaminants. It is recommended to sweep blast concrete surface before coating. If blast cleaning is not feasible, acid etching should be done using 05-10% Hydrochloric acid solution. After acid etching, the surface shall be flushed with fresh water to remove any residual acid and the surface should be allowed to dry perfectly.
Surface Preparation of Non Ferrous Metals
All surfaces should be clean, dry and free from oil/grease and other contaminants. If any corrosive salts are present they should be removed by abrading. Before coating, one thin coat of etch primer should be applied so as to provide a key for protective coat. Aluminium and galvanized (zinc) surfaces are normally coated by an oxide layer which inhibits the adhesion of coating. Incase of galvanized steel, white zinc corrosion products should be removed by high pressure fresh water washing (minimum 2500 psi) or fresh water washing with scrubbing. The galvanized surface should then be etched primed to passivate the surface and provide a key for further coating. For great blasting aluminium, use non-metallic abrasives like garnet.
Surface Preparation of Steel
Surface preparation of steel is carried out by various methods as under.
Degreasing
This is primarily done to remove oil, grease and other contaminants by solvents. Contaminants are removed by wiping the surface with rags soaked in a solvent, followed by wiping dry with clean rags. Surface can be checked for grease by sprinkling it with water. If pearly drops are formed, the surface is still greasy and needs a further treatment with degreaser. If the water flows out evenly, no grease is left on the surface.
Hand Tool Cleaning
It is a method of preparing surface with the help of emery paper, wire brush, scraper, chipping hammer, chisel, etc. to remove loosely adhering rust, mill scale or other contaminants. However, this method is incomplete and always leaves a layer of tightly adhering rust on the surface which will lead to failure of a coating in the long run.
Power Tool Cleaning
This is a better method of cleaning as compared to hand tool cleaning. In this method power tools like wire brush, disc sanders, grinders, etc. are used to carry out the job faster and is less laborious. However it will not remove tightly adhering rust /mill scale. While using power wire brush care should be taken not to polish the metal surface as this will reduce adhesion of coating.
Flame Cleaning
This is generally more effective method than power tool cleaning. Cleaning is carried out by oxyacetylene torch, hot air guns, etc. This method is generally used to remove old paint and varnishes. While cleaning care shall be taken not to damage/burn the substrate.
Abrasive Blast Cleaning
This is by far the most effective method for removal of mill scale, rust and old coating. In this method, an abrasive such as sand, grit or shots is injected into a rapidly moving air stream and directed at high velocity against the surface. Prior to blasting, surface should be degreased and all weld spatters removed. The profile of the surface after blasting depends on type of abrasive used, air pressure and blasting technique. Too low a profile may not provide a sufficient key for a coating, while too high a profile may result in uneven coverage of sharp peaks leading to premature coating failure, particularly for thin coatings such as blast primers. All residues shall be removed completely after blasting. For critical application, residues shall be removed by vacuum cleaning or blowing the surface with dry and oil free compressed air.The following table gives roughness profiles obtained by various types of abrasives.
| Type of abrasive | Mesh Size | Maximum profile height |
|---|---|---|
| Very fine sand | 80 | 37 microns |
| Coarse Sand | 12 | 70 microns |
| Iron Shot S 230 | 18 | 75 microns |
| Iron Shot S 390 | 14 | 90 microns |
| Iron Grit G 16 | 12 | 200 microns |
| Copper Slag (1.5 – 2.0 mm grain size) | - | 75/100 microns |
Note:
In many countries shot blasting by send is prohibited.
Wet Abrasive Blast Cleaning
This is a minor variation of abrasive blast cleaning process. Here the abrasive is mixed with water instead of air before being thrown at the surface. The water is mixed with rust inhibitor to prevent rusting of the surface immediately after blasting. This method largely helps to overcome dust hazards and associated health problems.
Blast Cleaning Standards
There are four commonly used visual grades for blast cleaning and the approximate equivalents between the various international standards are as follows.
| Description | White Metal |
Near White Metal |
Commercial Blast |
Brush Off Blast |
|---|---|---|---|---|
| Swedish Standard SIS 05 5900-1967 / ISO 8501-1:1988 | Sa 3 | Sa 2 1/2 | Sa 2 | Sa 1 |
| British Standard BS 4232 | First Quality | Second Quality | Third Quality | - |
| Steel Structures Painting Council (USA). Now The Society for Protective Coatings. | SSPC-SP5 | SSPC-SP10 | SSPC-SP6 | SSPC-SP7 |
| National Association of Corrosion Engineers (USA). | NACE 1 | NACE 2 | NACE 3 | NACE 4 |
| Shipbuilding Research Association of Japan, SPSS. | JA Sh 3 or JA Sd 3 |
JA Sh 2 or JA Sd 2 |
JA Sh 1 or JA Sd 1 |
- |
Level of Cleanliness
The following methods have been established by the Steel Structures Painting Council (USA), now The Society for Protective Coatings and others as levels of cleanliness prior to coating.
SSPC-SP-1 / Solvent Cleaning
Removal of all detrimental foreign matter such as oil, grease, dirt, soil, salts, drawing and cutting compounds, and other contaminants from steel surfaces by the use of solvents, emulsions, cleaning compounds, steam or other similar materials and methods which involve a solvent or cleaning action.
SSPC-SP-2 (Swedish Standard St 2) / Hand Tool Cleaning
Removal of all rust scale, mill scale, loose rust and loose paint to the degree specified by hand wire brushing, hand sanding, hand scraping, hand chipping or other hand impact tools or by a combination of these methods. The substrate should have a faint metallic sheen and also be free of oil, grease, dust, soil, salts and other contaminants.
SSPC-SP-3 (Swedish Standard St 3) / Power Tool Cleaning
Removal of all rust scale, mill scale, loose paint, and loose rust to the degree specified by power wire brushes, power impact tools, power grinders, power sanders or by a combination of these methods. The substrate should have a pronounced metallic sheen and also be free of oil, grease, dirt, soil, salts and other contaminants. Surface should not be buffed or polished smooth.
SSPC-SP-4 / Flame Cleaning
Removal of all loose scale, rust and other detrimental foreign matter by passing high temperature, high velocity oxy-acetylene flames over the entire surface, followed by wire brushing. Surface should also be free of oil, grease, dirt, soil, salts and other contaminants.
SSPC-SP-5 (Sa 3, NACE 1) / White Metal Blast Cleaning
Removal of all mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels. A White Metal Blast Cleaned Surface Finish is defined as a surface with a gray-white, uniform metallic color, slightly roughened to form a suitable anchor pattern for coatings. The surface, when viewed without magnification, shall be free of all oil, grease, dirt, visible mill scale, rust, corrosion products, oxides, paint, or any other foreign matter.
SSPC-SP6 (Sa 2, NACE 3) / Commercial Blast Cleaning
Removal of mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree specified. A Commercial Blast Cleaned Surface Finish is defined as one from which all oil, grease, dirt, rust scale and foreign matter have been completely removed from the surface and all rust, mill scale and old paint have been completely removed except for slight shadows, streaks, or discolorations caused by rust stain, mill scale oxides or slight, tight residues of paint or coating that may remain; if the surface is pitted, slight residues of rust or paint may by found in the bottom of pits; at least two-thirds of each square inch of surface area shall be free of all visible residues and the remainder shall be limited to the light discoloration, slight staining or tight residues mentioned above.
SSPC-SP-7 (Sa 1, NACE 4) / Brush-Off Blast Cleaning
Removal of loose mill scale, loose rust, and loose paint, to the degree hereafter specified, by the impact of abrasives propelled through nozzles or by centrifugal wheels. It is not intended that the surface shall be free of all mill scale, rust, and paint. The remaining mill scale, rust, and paint should be tight and the surface should be sufficiently abraded to provide good adhesion and bonding of paint. A Brush-Off Blast Cleaned Surface Finish is defined as one from which all oil, grease, dirt, rust scale, loose mill scale, loose rust and loose paint or coatings are removed completely but tight mill scale and tightly adhered rust, paint and coatings are permitted to remain provided that all mill scale and rust have been exposed to the abrasive blast pattern sufficiently to expose numerous flecks of the underlying metal fairly uniformly distributed over the entire surface.
SSPC-SP-8 / Pickling
Removal of all mill scale, rust and rust scale by chemical reaction (using sulfuric, hydrochloric or phosphoric acids), or by electrolysis, or by both. It is intended that the pickled surface shall be completely free of all scale, rust, and foreign matter. Furthermore, the surface shall be free of unreacted or harmful acid or alkali.
SSPC-SP-9 / Weathering Followed By Blast Cleaning
Weathering to remove all or part of the mill scale followed by one of the blast cleaning standards.
SSPC-SP-10 (Sa 2-1/2, NACE 2) / Near-White Blast Cleaning
Removal of nearly all mill scale, rust, rust scale, paint, or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree hereafter specified. A Near-White Blast Cleaned Surface Finish is defined as one from which all oil, grease, dirt, mill scale, rust, corrosion products, oxides, paint or other foreign matter have been completely removed from the surface except for very light shadows, very slight streaks or slight discolorations caused by rust stain, mill scale oxides, or light, tight residues of paint or coating that may remain. At least 95 percent of each square inch of surface area shall be free of all visible residues, and the remainder shall be limited to the light discoloration mentioned above.
Coating Application
Liquid coatings can be applied by brush, roller, spray or airless spray methods. Coatings in putty form can be applied by applicators. While coating, relative humidity should not be above 65%. In general if the surface to be painted dries within 10-15 minutes after moistening, it may be considered to be all right for coating. Take care of drying and curing time of a coating depending on surrounding temperature. Check minimum application temperature of the coating. It will not cure below it. Brief detail on various application methods is as under.
Brush Application
Brush Application is a relatively slow procedure, but is generally used for decorative paints and for coating small areas. It is particularly suitable for coating complex small areas where the use of spray method leads to considerable losses due to overspray. The application is not suitable for high film built (thick coating). Coat with the brush at an angle of 45º to minimise brush marks. Avoid using a new brush for a final coat as new brushes have a tendency to shed bristles.
Roller Application
This is a faster method than brush application on large even surfaces. In this method control of film thickness is not easy. The method is not suitable for high film build.
Conventional Spray
This is a widely accepted and rapid method of coating application in which the material is atomised by an air stream. In this method it is essential to use the correct combination of air volume, air pressure and material flow to get good atomisation and a coating film free from defects. The method is not suitable for high build film as most coating material has to be thinned to a suitable viscosity for satisfactory atomization.
Pressure Pot Spray
This is a minor variation of conventional spray method. In this method, coating material is delivered to spray gun from a tank (Pot) instead of a gravity feed cup attached to spray gun. Compressed air is used to force coating material from the tank to the spray gun through a hose. Pressure pot system is used where large quantity of coating material is to be applied. This method enables the gun to be turned to any angle without spilling of material and also saves time due to elimination of frequent filling of gravity feed cup.
Airless Spray
Unlike conventional spray and pressure pot method air is not mixed with the coating material to form a spray, hence the name airless spray. Atomisation is achieved by forcing the material through specially designed nozzles or tips, by hydraulic pressure. The method is used for high build coatings.The tips through which the material is forced to achieve atomisation are precisely constructed from tungsten carbide. The atomised “fan” (spray width) of material is produced by a slot ground on the face of the orifice. Various orifice sizes together with different slot angles are available.Approximate size of orifice to be selected for various wet film thickness is as under.
| Orifice Size in mm | Wet Film Thickness in microns |
|---|---|
| 0.02 – 0.03 | Up to 50 |
| 0.03 – 0.04 | 100 – 200 |
| 0.04 – 0.07 | More than 200 |
| 0.10 – 0.15 | For Mastics |
Putty Application
Putty is applied using trowel, spatula, flexible applicator, putty knife, etc. The method is generally used to rebuild damaged component or to enhance life of a component. The material shall be applied by pressing it down well to force out any entrapped air and to insure intimate contact of the material with the surface.
Safety Precautions – Storage and Application of Coating Materials
Safety precautions to be taken during storage and coating application are as under. They are not intended to be exhaustive and do not cover all eventualities.
Fire and Explosion
The majority of paints contain flammable organic solvents. In view of this, naked flames, cigarettes or matches should not be used near an area where paint is being applied or stored. Do not extinguish fire involving paint with water as paint solvents will float on water and will help to spread the fire. Use a dry chemical, foam or CO2 extinguisher.
Skin and Eye Contact
Wear personal protective equipment like gloves, safety glasses, working clothes, etc. If required a non-greasy barrier cream may be applied on all exposed skin. If any material splashes in the eyes, flood them immediately with fresh water and immediately seek medical advice. If material splashes on skin, remove it with soap and water or an industrial cleaner. Do not use solvent.
Inhalation
Ensure that adequate ventilation is available to remove solvent fumes. Solvent fumes are heavier than air and they push breathable air upwards. If dizziness, drunkenness or headaches are experienced, this could be due to solvent fumes. In such situation move into fresh air and do not return until the ventilation has improved. If breathing fumes result in the collapse of a person, he should be carefully moved into fresh air and allowed to recover gradually. Forced exercise is not advisable. If spaces are difficult to ventilate efficiently, wear an air fed hood/musk. Wear dust cartridge respirator during spraying to prevent inhalation of coating particles mist in a well ventilated spaces.
Ingestion
Food and drink should not be consumed, stored or prepared in areas where coatings are stored or being applied. In the case of accidental ingestion, medical attention should be obtained at once.
