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Proper surface preparation is of great importance in obtaining the optimum film performance. This page describes the initial surface treatment for steel plates, secondary surface treatment for fabricated steels, and the application of repair paint.
A. Initial Surface Treatment for Steel Plates
The following initial surface treatment is to be applied to steel plates.
1. Oil or grease shall be removed by wipping or scrubbing the steel with clean rags or brushes wetted In solvent.
2. Corroding salts, such as chlorides and sulphates etc., on the steel's surface shall be rinsed-Off with fresh water. Water and moisture shall be removed by wiping the steel using dry rags or by drying the steel with hot forced air.
3. All mill-scale, rust, rust scale, paint marks or foreign matter shall be removed by shot-blasting, grit-blasting, or sandblasting.
4. Before shop primer is applied, dust, sand residue, crushed steel shot or any grit and all other contaminants must be removed from the surface using a vacuum cleaner, air blower, etc.
B. Secondary Surface Treatment For Fabricated Steel And The Application Of Repair Paint
Defective areas with damage and rust owing to gas cutting, welding, and stress relieving measures must be cleaned by blasting or with power tool cleaning. Degreasing and washing may also be necessary to clean the surface before the subsequent coats are applied. To do so, follow the steps below:
1. Remove corroding salts, chalk marks, soil or other contaminants and foreign matter by brushing the steel with a stiff fibre or wire brush or a combination of both.
2. Deposited oil and grease must be removed using solvent.
3. Use a power tool to clean rust and damaged paint film from areas suffering from stress relief, heat treatment.
4. Use a blast cleaner or power tool to remove weld flux slugs, weld metal spatters, weld flux fume deposits, rust and damaged paint film in welded areas.
5. Use a vacuum cleaner to remove dust, sand residue, and other contaminants.
Table 1 below shows the main methods of pre—treatment which can be used to remove rusted paint:
METHOD |
RESULT |
Blast Cleaning |
Ideal |
Mechanical Wire Brushing |
Acceptable |
Mechanical Disk Sanding |
Acceptable |
Needle Chipping |
Fair |
Mechanical (air powered) scraping |
Fair |
Hand Brushing |
Poor |
Hand Scraping |
Poor |
Table 2 below shows definitions of surface preparations according to ISO 8501-1.
PREPARATION BY SCRAPING AND WIRE BRUSHING |
St 2 |
Through scraping and wire brushing/machine brushing / grinding / etc., the treatment must remove loose mill scale, rust and foreign matter. Finally, the surface is cleaned with a vacuum cleaner, clean dry compressed air or a clean dry brush. It should have a faint metallic sheen. The appearance must correspond to the prints designated ST2.0 |
St 3 |
Very thorough scraping and wire brushing/machine brushing/grinding/etc. Surface preparation as for ST2.0 but mush more thoroughly treated. After removal of dust, the surface must have a pronounced metallic sheen and correspond to the prints designated ST3.0 |
PREPARATION BY BLAST CLEANING |
Sa 1 |
Light blast cleaning. Loose mill scale, rust and foreign matter must be removed. The appearance shall correspond to the prints designated SA1.0 |
Sa 2 |
Thorough blast cleaning. Almost all mill scale, rust and foreign matter must be removed. Finally the surface is cleaned with a vacuum cleaner, clean dry compressed air or a clean dry brush. It must then correspond is appearance to the prints designated SA2.0 |
Sa 2.5 |
Very thorough blast cleaning. Mill scale, rust and foreign matter must be removed to the extent that the only traces remaining are slight stains in the form of spots or stripes. Finally the surface is cleaned with a vacuum cleaner, clean dry compressed air, or a clean dry brush. It must then correspond in appearance to the prints designated SA2.5 |
Sa 3 |
Blast cleaning to pure metal. Mill scale, rust and foreign matter must be removed completely. Finally the surface is cleaned with a vacuum cleaner, clean dry compressed air, or a clean dry brush. It must then have a uniform metallic colour and correspond in appearance to the prints designated SA3.0 |
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