Code of Practice for Painting of Ferrous Metals in Buildings, Part 1: Pretreatment
1971 Edition

Recommended mechanical methods include hand scraping, chipping, wire brushing, and using abrasive papers or steel wool primarily for loose rust or older rust scales. Power-driven wire brushes offer improved cleaning but may not entirely remove firmly-attached mill scale. Flame cleaning is applicable in certain site conditions but less common for mill scale elimination. Sand or shot blasting stands out as the most effective mechanical approach for completely removing mill scale and rust, providing the best surface preparation for painting. For heavily rusted old steel, initial scraping or chipping is advised before blasting.

Sand-blasting is regarded as the most thorough cleaning technique, fully eliminating rust, scale, and mill scale to yield a consistent silver-grey surface ideal for painting. Although it incurs higher initial costs, comparable to a premium four-coat paint system, it significantly reduces maintenance by preventing corrosion over longer periods. In contrast, commercial blast cleaning may leave residues or 'shadows' and is less comprehensive. Sand-blasting requires air compressors and protective measures against silica dust inhalation, which adds to operational considerations. Overall, sand-blasting offers superior surface cleanliness and coating longevity at a higher upfront investment.

Key safety protocols include operators wearing safety helmets equipped with forced-air supply, while others in the vicinity should use filtered air respirators. Protective goggles are mandatory near blasting zones. Dust management involves conducting blasting downwind to minimize inhalation risks. When blasting tank roofs, covering all openings with clean muslin cloth helps prevent abrasive ingress yet permits venting. In flammable environments, grounding and bonding of the nozzle, hose, sand hopper, tank, and compressor are critical to prevent static spark ignition. These measures ensure personnel safety and mitigate fire hazards during operations.

The standard highlights phosphate treatment as the most effective chemical pretreatment, typically applied via hot-tank processes such as Granodizing, Bonderizing, or Walterizing, which significantly improve paint adherence and inhibit rust formation. Cold phosphating is also mentioned but is less effective and suited for on-site applications. Etching or wash primers deposit acid-bound resinous films on metal surfaces, enhancing bonding of subsequent paint layers and can be applied by brushing, spraying, or dipping. Following pretreatment, immediate priming is essential, and damaged factory-pretreated surfaces should be degreased and touched up with appropriate primers before repainting.

Painting should be applied immediately after surface preparation to ensure maximum protection. Delays, especially in humid or coastal environments, can lead to rust formation within hours, which undermines paint adhesion and corrosion resistance. Surfaces must be free from rust, scale, moisture, grease, and salts before painting. If any delay occurs beyond a few hours, especially in aggressive atmospheres, the surface requires re-cleaning to remove newly formed rust before painting. Immediate priming after pretreatment is also emphasized. Prompt application minimizes corrosion risk and prolongs coating life.