Painting is the most common way to protect steel structures. Before carrying out painting work, surface preparation must be properly taken. Here after is surface preparation procedure for steel structure painting.
I/ Steel surface treatment:
1. Welded areas shall be checked to verify any defects that may affect the protection quality of coating paint.
2.Sharp edges shall be grinded to be round, smooth. Other imperfect weld or slag shall be treated by grinding machine or sand paper.
3. Noncontinuous welding line shall be filled again by welding.
II/ Surface preparation:
1. Thoroughly remove oil and grease out of the surface using solvent or other proper methods.
2. Thoroughly remove all "evidence" of salt and contaminants.
3. Sand blast to achieve standard SA 2.5 (surface is free from oil, grease and other contaminants)
4. Remove all the remaining during blasting by brush, pressed air or vacuum cleaner. Avoid re-contamination caused by clothes or hand touching.
5. Sand blasting shall be taken from one area to another, so that cleaned areas must be rust preventive and painted immediately before getting rust again. Surface ready for painting must be completely clean. No oxidized or contaminated is visible.
6. Surface temperature is at least 3 degrees above the dew point to avoid water condensing. (Dew point is depended on surface temperature and humidity)
7. Areas that cannot reach with blasting nozzle or less important can be mechanically treated with grinding machine
III/ Inspection and evaluation for steel surface preparation
1. Oil and grease check
Oil and grease check shall be carried out a 2-3 locations per square meter and in 40-50% out of prepared area, as following:
Drip some drops of gasoline onto checked area. Wait for 10-15 seconds then adsorb the remaining gasoline with a piece of filter paper. Drip some other drops of gasoline onto another piece of filter paper. Wait and check the two dry stains on paper by naked eyes. If two colors look the same the surface is accepted as free from oil and grease.
2. Dust check
Dust check shall be carried out on whole prepared surface. Use a magnifier with 6 times magnification to survey. No visible dust is okay.
3. Cleanness check
Dust check shall be naked eyes or a magnifier according to cleanness levels. It also can be checked by comparing to standard images in ISO 801-1:1998
Another method of checking: Before surface preparation, take a sample that meets the cleanness of requirements. Cover the sample with transparent coating. Use the sample as standard for control.
Fast check: Cleanness check can be quickly conducted by wiping the surface with a white dry clean cotton. If the cotton becomes dirty, the surface is not clean. If the cotton becomes wet, the surface is either still wet or not free from oil.
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