Painting Metallic Works Procedure: Blasting, Painting & Repair

1.   BLASTING

1. Prior to painting, the surface for application is thoroughly cleaned of any material that may result in premature failure of the painting. Blasting or wire brushing will be completed in accordance with the customer specifications and manufacturers recommendations. 

2. Before using compressed air source for the surface preparation, the quality of air at the Blast nozzle should determined by blowing in to a piece of absorbent paper (Blotter). 388 Kpa(50 psig). The nozzle distance should be approximately 450mm -600mm (18-24 inches) the air flow should be maintained for one(1) minute to check for any contamination of oil or moisture is present.

3. Before blast cleaning takes place, deposits of oil , grease and salt must be removed. Clean and dry compressed air free of oil, must be used for blasting. Abrasives used should have the following characteristics:

• The abrasive has to be sharp and angular (so that the casting rate is high)
• 30-60 mesh can produce surface of 40 to 60 microns.

• It should be having a hardness of 6 to 7 Mohs scale ( the abrasive has the capability to penetrate hard to steel)
• All abrasive should be cleansed so that high productivity is guaranteed due to kinetic energy is principle.
• It should have no chloride content (premature paint failure is avoided). The allowable limit of chloride content should not exceed 25ppm.
• Low silica content  1% free silica maximum.
• A grit size should be in the range 0.3 - 1.5mm (12-60mils) in order to have a good production rates and achieving specified standards and especially when pitted surfaces.

4. Blast System should have been operated for the last 5 min. under actual field condition prior to performance of his Blotter test. The test is also made after any interruption of the air compressor operation. The air is used only if the Blotter test indicates no visible contamination.      

5. Ensure the Air supply compressor, Air receiver, Pressure blast machine, Blasting hose and Airless spray equipment are in good condition and should be certified for use and a current test certificate available. Blasting nozzles should contain an operational deadman remote control valve. Painting equipment report should be used for verifying equipment suitability.

6. Hoses, pumps and accessories should never be operated at pressures exceeding their rated pressure.

7. Cover or otherwise protect all sensitive equipment, instruments, plastic, glass, valve stems, and other adjacent items from damage or contamination from blasting abrasives.

8. After blasting, surface dust must be removed. In open conditions, blowing with dry compressed air the blasting kettle is satisfactory. Tanks however require careful cleaning; normally using vacuum cleaners to remove all grit dust particles 

2.  PAINTING

           

1. Identify and quantify the scope on the required painting or its repair for the rejected painting.  Identify the painting type and specification requirements.

2. Check and verify acceptable RH (relative humidity), dew point & substrate temperatures and air temperatures for the coating system to be applied.

3. Ensure that safety requirements defined in material safety sheets are complied with throughout the course of each activity.

4. Paints should not be used from containers showing signs of leakage.

5. Paint containers should be clearly marked with batch nos. and the date of manufacture.  Any paints exceeding the recommended shelf life should be discarded.

6. Ensure all relevant painting data sheets and work procedures / specifications are available to the relevant personnel.

7. Ensure that painting personnel are suitably qualified and approved by the client for the intended work scope (when required).Qualification requirements for painters, blasters and crew supervisor.

8.  Obtain the necessary work permits prior to commencing work.

9. To ensure satisfactory results on painting or its repair, verify the adequate   clearance around the item to be coated or around the full area to be repaired, respectively.

10. All painting materials used on a single item should be from one manufacturer finishing coat should be from one batch no. only.    

11.  The type of abrasive to be used should be determined by the degree of   cleanliness and surface profile requirements for the particular painting system.  In all cases, the   manufacturer's painting data sheet and customer specification must be referenced.

12. Paint should be thoroughly mixed before adding thinner, observing induction time in accordance with the manufacturer’s recommendations. Power mixers should be used for two-pack paints and all other types with quantities above 5 Liters.

13. Paint should be applied by one of the following methods.

• Brush
• Roller
• Airless Spray
•  Air Spraying (conventional)

14. Brush shall be applied in panting, wherever possible, be applied at 90° to the previous paint.

15. An airless spray gun shall be use for a paint stream by ejecting it at a high pressure from specially designed tip.

16. A conventional spray gun forms paint droplets by mixing air with the paint stream but turbulence will be rather higher than with airless spray. 

17. To ensure the paint is fully dispersed in fine droplets, its viscosity must be low.

18. The area sprayed should be within a comfortable distance of the operator to avoid a long traverse of the jet or arcing of the gun.

19. It may be necessary to perform solvent, detergent and/or potable water wash, prior to repair, after consideration of the following:

• Environment and system application (immersion service, corrosive, high   temperature etc.).
• Painting system / primer used.
• Time lapse between initial coating and repair.
• Amount of visual contamination present.
• Customers painting specification.

20. Painting Should be applied observing the WFT and DFT requirements for the particular system. Repainting intervals should be in accordance with data sheets.

21. Curing of all painting should be in accordance with the manufacturer’s recommendations and is dependant upon parameters such as application method, temperature, % thinner etc.                                           

3.   REPAIR     

1. For repair areas with a dimension equivalent to or less than 100mm x 100mm the following methods of surface preparation should be employed, unless otherwise specified in the Customer’s Specification or coating data sheets.

• a) Grind to a rough metal surface using at least an 80 grit disc grinder (area at least one meter).
• b) Spot blast an area at least one meter to bare steel achieving a finish equal to that specified on the coating data sheet.

2. Adjacent pipe paintings should be feathered back to prevent stepping and roughened using the sand paper to provide a suitable key. The key should extend at least 50mm into the existing coating.

3. For Fusion Bonded Epoxy painted items, a two-pack repair kit should be used as recommended by the Fusion Bonded Epoxy painting manufacturers.

4. Inspection requirements should be in accordance with the original procedure including requirements for Request for Inspection (RFI).

Would you like to have a full copy of its native file (word & pdf copy)? This Construction Procedure for Painting Metallic Works may includes still the following;

1. Purpose

2. Scope

3. Applicable Codes and Standards

4. Responsibilities

5. Material Storage Condition

6. Weather/Ambient Condition

7. Procedure {this ariticle}

8. Inspection

9. Quality Control and Assurance

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