Surface Preparation
Introduction:
The effective life of paint coated on steel surface depends largely on steel surface preparation before painting. Surface preparation must be done before coating shop primer or primer.
Firstly, remove the oxide, rust and foreign materials.
Secondary surface treatment: Remove rust and foreign materials. Remove rust and foreign materials from the surface which have been burshed shop primer or primer before coating.
The method of steel surface preparation as follows:
The traditional method of Rotating steel brush treatment is effective to treat the weld, but not conducive to removal oxide. The major disadvantage is that it can not completely remove the corrosion, and always polished surface with oil after sweeping handle.
Eradicative Treatment:
Eradicative treatment or mechanical treatment is usually combined with steel bursh. The method is very suitable for traditional or specially formulated paint to partial repair, but ,it is not suitable for epoxy or chlorinated rubber paint to pretreatment. The method also can be used to remove rust and reducee the work of subsequent blast.
Pin Hammer Treatment:
Remove rust and paint from the angle and gap to get a clean surface and shape.
Flame Treatment:
Flame treatment is use a combustion equipment which is burning acetylene, propane and oxygen to remove rust. It can remove most of oxide, but the corrosion can only be reduced to a lower level. Therefore, this method can not meet the requirements of modern painting
Rotary Disc Grinding:
Rotary disc grinding work with a sticky, high speed rotating disk. The method is used
to partial repair or remove barnacles.The grinding quality is greatly improved, and
can reach a higher pretreatment standards.
Sweep Blasting:
The method is to roughening the surface of coated primer by manual, it can remove almost all surface dirt (except for oil and rust stains).
A: Mild cleaning
Objective: To roughenss the complete coating.
Abrasive: 0.2-0.5mm
B: Heavy cleaning (about Sa1 level)
Objective: To remove the layer of paint from the solid coating region.
Abrasive: 0.2-0.5mm or 0.2-1.5mm
Blasting Treatment:
Blasting treatment is to spray particles to the surface which is to be treated with high speed. It can be used hand or wheel to eject. The method is the most thorough method to remove rust. Centrifugal blast cleaning, compressed air and vacuum blast cleaning blast cleaning is a very common method.
Shot:
Pills is semicircular solid, Pills with a “tail” or irregularly shaped can not exceed the specified minimum. It is necessary to check the suitability of the primer used by shot treatment.
Angular Sand:
Abrasive particles have good edges, sharp cutting, and no a semi-circular particles. If no otherwise stated, mineral angular sand generally should be used.
Water (Sand) Blasting:
Ultra high pressure water washing:
Pressure: higher than 1700Pa
Application: Complete remove all coatings and rust. The effects can match dry abrasive blasting, but the flash rust will appeared after drying.
High pressure water washing
Pressure: 700-1700Pa
Application: Remove most of old coating and rust. It difficult to remove tightly adherent old coating and rust.
High pressure water washing
Pressure: 350-700Pa
Application: It can remove loose coating, rust and other materials, but can not remove thick rust and appear metallic luster.
Low pressure water washing
Pressure: 350Pa
Application: Remove salt loose dirty. Mainly used for cleaning the surface.
Low pressure water abrasive washing
Pressure: 6-8kg/cm2
Clean speed: 10-16m2/h, depending on the contamination.
Application: Saving abrasive, reducee dust, salt and avoid sparks. The effects can match dry abrasive blasting, but the flash rust will appeared after drying.
Steam washing
Pressure: 100-120 kg/cm2
Application: Remove drity of water-soluble or water emulsion. Relative to the water washing method, the substrate after steam cleaning dry quickly.
National Standard:
GB/T 8923.1-2011 Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings.
GB/T 8923.2-2008 Preparation grades of previously coated steel substrates after localized removal of previous coatings.
GB/T 8923.3-2009 Preparation grades of welds, edges and other areas with surface imperfecions.
GB/T 8923.4-2013 Initial surface conditions, preparation grades and flash rust grades in connection with high-pressure water jetting.
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The rating was accurate before blasting and cleaning the surface. |
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Sunrui adopt GB/T 8923-2011. This standard specifies the following |
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corrosion rating. |
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A, Steel surface is fully covered with oxide and almost no rust. |
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B, Steel surface has occurred corrosion and the partial oxide has been |
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peeling. |
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C, Steel surface has a small amount of pitting corrosion oxide has |
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been spalling due to corrosion and can be scraped. |
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D, Steel surface has been widespread occurred pitting corrosion, and |
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oxide has been spalling due to corrosion. |
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The |
Main Level |
The standard GB/T 8923.1-2011 includes seven treatment levels. As |
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of |
Surface |
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Treatment |
the following. |
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Hand and Power Hand or power tools treatment, such as scratches, steel brush
Tools Treatment handling, mechanical processing and polishing, show as “St”.
St2 Thoroughly Cleaning by Hand and Power Tools
Any thick rust should be knocked out and oil, grease dust should be removed before adopt hand and power tools clean. The steel surfaces should be clean and free from dust and residue after cleaned.
St3 Very Thoroughly Cleaning by Hand and Power Tools
Rust should be more thorough than St2, the surface of the substrate should have metallic luster.
Blasting
Cleaning
Blasting cleaning surface treatment show as “Sa”.
Any thick rust should be knocked out and oil, grease dust should be removed before blasting cleaning.
The steel surfaces should be clean and free from dust and residue
after cleaned.
Sa1 Remove rust by mild jet or projectile
The steel surface should be no visible oil and dirt, and no loose
oxide, rust and old coatings.
Sa2 Remove rust completely by jet or projectile. The steel surface should be no visible oil and dirt, and oxide, rust, old coatings has basically cleared, the residue should be firmly attached.
Sa2.5 Remove rust very thoroughly by blasting cleaning
The steel surface should be no visible oil and dirt, any traces of
residue should be the minor spots of dot or stripes
Sa3 Remove rust by blasting cleaning to appeare metallic luster
The steel surface appeared clean and no visible oil, dirt, oxide rust,
old coatings. The surface should show uniform metallic luster.
Flame Cleaning
Flame cleaning surface treatment show as “F”.
Any thick rust should be knocked out before flame cleaning.
The steel surfaces should be clean by steel brush handling.
F1 The steel surface should be no visible oil and dirt, any traces of
residue should be colour change of the surface.
Proposal:
Photos in GB/T 8923.1-2011 is only a description, it can not represent all of the treatment status. Because the photos do not reflect the visual processes of cleaning operation (such as colorless pollution).