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The objective of applying a paint coating is to provide a film that will protect and decorate the surface being painted. The success of any paint application is generally governed by the following factors :
- Surface Preparation
- Film Thickness
- Methods of Application
- Conditions during Application
Surface Preparation
How any paint coating performs, depends on the correct and thorough preparation of its surface. Given below are some of the recommended surface preparation techniques commonly followed.
Surface : Steel
- Degreasing - It is essential to remove all traces of oil, grease, drilling/ cutting compounds and any other surface contaminants. The most common method is by solvent washing, followed by wiping dry with clean rags. Recommended procedures are described in SSPC-SP1.
- Hand tool cleaning - It is a method of preparing steel surfaces by the use of non-power hand tools. Loosely adhering mill scale, rust and old paint coatings may be removed from steel by chipping, scraping, hand wire brushing and emery paper cleaning. However, these methods are incomplete and always leave a layer of tightly adhering rust on the steel surface. Procedures for hand tool cleaning are described in SSPC-SP2 and should be to ST2 grade of Swedish Standard or ISO 8501-1: 1988.
- Power Tool Cleaning - For the removal of loosely adhering mill scale, paint and dust, this is more effective and less laborious method than hand tool cleaning. It will not however remove tightly adhering mill scale. Power wire brushes, needle guns grinders and disc sanders are generally used. Care should be taken, particularly when using power wire brushes, not to polish the metal surface, as this will reduce the key for subsequent painting. Methods are described in SSPC-SP3 and should be to ST3 grade of Swedish Standard or ISO 8501-1: 1998.
- Flame Cleaning - Generally more effective than power tool cleaning, but has the inherent risks of fire and explosion, and of possible distortion of light gauge steel.
- Blast Cleaning - By far the most effective method for removal of mill scale, rust and old coating by using abrasives such as sand, grit or shots directed at high velocity against the surface. Prior to blasting, steelwork should be degreased and all weld spatters removed. Weld seams and sharp edges should be ground down. This is because paint coatings tend to run away from sharp edges, resulting in thin coatings and reduced protection. The grade of blasting suitable for a particular coating specification depends on a number of factors, the most important of which is the type of coating system selected. There are four commonly used grades for blast cleaning, and the approximate equivalents between the various International Standards are as follows :
| |
Steel Structures Painting Council (USA) |
National Association of Corrosion Engineers (USA) |
British Std. BS 4232 |
Swedish Standard SIS 05 5900 - 1967 / ISO 8501-1 : 1988 |
Shipbuilding Research Association of Japan - SPSS. |
| White Metal |
SSPC - SP 5 |
NACE # 1 |
1st Quality |
SA3 |
JA Sh 3 or
JA Sd 3 |
| Near White Metal |
SSPC - SP 10 |
NACE # 2 |
2nd Quality |
SA 2½ |
JA Sh 2 or
JA Sd 2 |
| Commercial Blast |
SSPC - SP 6 |
NACE # 3 |
3rdQuality |
SA 2 |
JA Sh 1 or
JA Sd 1 |
| Brush Off Blast |
SSPC - SP 7 |
NACE # 4 |
- |
SA 1 |
- |
The profile of roughness obtained during blasting is important, and will depend on the abrasive used, the air pressure and the technique of blasting. 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. An important ancillary to the blast cleaning process is the removal of abrasive and dust from the surface before paint application. This process must be carried out efficiently. The following table gives a brief guide to typical roughness profiles obtained using various types of abrasives :-
| |
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 50 |
25 |
83 microns |
| Iron Grit G 16 |
12 |
200 microns |
| Copper Slag (1.5-2mm grain size) |
- |
75/100 microns |
Source :
Table below gives approximate open-air blast cleaning rates and abrasive computation on neatly fabricated steel using 3/8-inch venture nozzle with 100 psi nozzle pressure to a SA 2½ near white condition.
| Types of Abrasive
|
Abrasive Consumption
|
Production Rate
|
Comments
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| Silica Sand 16/40 mesh
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12.69 Kg/Sq.mt
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0.44 sq.mt/min
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Dusty - Not reusable
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| Garnet 36 grit
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17.58 Kg/Sq.mt
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0.33 sq. mt/min
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Very little dust - Reusable
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| G. 40 Steel Grit
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26.85 Kg/Sq.mt
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0.28 sq. mt/min
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No dust - Reusable
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| Copper Slag 16/40 mesh
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15.13 Kg/Sq.mt
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0.41 sq. mt/min
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No dust - Reusable
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- Surface : Non Ferrous Metal/ Aluminium/ Galvanised Steel - All surfaces should be clean, dry and grease - free. If any corrosive salts are present they should be removed by light abrading. Before painting, one thin coat of etch primer should be applied so as to provide a key for further coats. Incase of galvanised steel, any white zinc corrosion products should be removed by high pressure fresh water washing or fresh water washing with scrubbing. The galvanised surface should then be etched primed to passivate the surface and provide a key for further paint coatings.
- Surface : Concrete/ Masonry/ Concrete Flooring - All surfaces should be clean, dry, and free from oil, grease and other contaminants, such as forming lubricants and curing components, which would affect adhesion of a paint coating. The moisture content of the concrete or masonry should be less than 5%. Painting over surfaces which have not sufficiently dried out, will result in blistering and flaking of the paint coating, as the trapped moisture gradually escapes. If a surface is persistently damp, which may be due to hygroscopic salts, the cause should be sought and corrected before attempting to apply paint. Laitence and loose surface powder formed on new concrete must be removed. The most preferable surface treatment for concrete is sweep-blasting. Cracks on the surface should be cut out and filled with a suitable filler prior to painting. If blast cleaning is not feasible then acid etching should be done using 10-15% Hydrochloric acid solution, followed by thorough flushing with fresh water to remove any residual acid. The surface should be allowed to dry perfectly before painting. Ensure that the concrete surface is cured for minimum 2 months before painting.
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