Surface Preparation
Core concepts
Surface preparation is the single largest determinant of coating performance. Candidates must know abrasive blast cleaning standards, surface profile, cleanliness, and the mechanisms by which contaminants cause premature failure. Understand the difference between surface cleanliness (removal of mill scale, rust, paint, and contaminants) and surface profile or roughness (anchor pattern that promotes mechanical adhesion).
Standards and methods
Blast cleaning grades are commonly described by visual standards (for example, white metal, near-white, commercial, and brush-off blast) and by abrasive blast standards such as SSPC-SP 5, SP 10, SP 6, and SP 7, or the ISO 8501-1 Sa grades. Power-tool cleaning (SP 3, SP 11) and hand-tool cleaning (SP 2) produce lower cleanliness and no meaningful profile. Solvent cleaning (SP 1) removes oil, grease, and salts but does not remove rust or mill scale. Profile is typically measured by replica tape or comparators and reported in mils or micrometers.
Contaminants and traps
Soluble salts (chlorides, sulfates) drive osmotic blistering and undercutting; they are detected by conductivity testing of a surface extract. Questions often blur the line between removing visible rust and removing invisible salts-blasting alone may not remove salts embedded in pits. Another trap: over-blasting to an excessive profile can cause thin spots and holidays at profile peaks, while too little profile causes poor adhesion. Watch for confusion between anchor profile and cleanliness; a surface can be visually clean yet salty.
Worked example / rule of thumb
Rule of thumb: prepare to the specified grade, verify profile with replica tape, and test for soluble salts when the specification requires it-do not assume a white-metal appearance means a salt-free surface. If a spec calls for near-white blast with a 2–3 mil profile, confirm both the visual grade and the profile number separately.