Selected Answers

From Stephen Bothello of Jotun UAE Ltd on January 16, 2018:
Generally, power tool cleaning will be specified f ...read more Generally, power tool cleaning will be specified for spot repairs of coating up to the condition of bare metal, where abrasive blasting is either not advisable (confined space, sensitive areas) or may create more work than intended (like over-blast of adjacent, sound coating areas, distorted edges that require feathering). Historically, abrasive blast cleaning method was found efficient (economics, more area covered at less man-hours) and effective (gives a definite surface profile that is critical for coating adhesion). The better the coating adhesion, the better will be the coating performance for most, if not all coatings. However, power tools based on latest advance technology used for SSPC -SP-3 are now closely contesting abrasive blast cleaning methods. Many power tool manufacturers claim to provide expected surface profiles, cover more unit area in time and demonstrate laboratory test results to confirm coating performance. However, the decision to apply the power tool to the entire surface or limited only to defective/ rusted areas will be taken by asset owners based on many factors. Foremost under consideration is the coating exposure environment, whether atmospheric (ISO 12944- C1 to C5), immersion or buried (ISO 12944-Im2/Im3). Note that IMO PSPC provides a limit for power tool cleaning up to 2% of surface area or less than 25 sq.m of contiguous damages for sea water ballast tank linings (new construction). This suggests a good guideline by the industry body for coatings subject to immersion exposures. For atmospheric areas, these limits can be stretched based on coating compatibility and aesthetics. The extent of the defective or rusted area is another critical consideration. Generally, for most coating manufacturers a degree of rusting up to and  below Ri3 per ISO 4628-3 (Rust Scale below 10 to 6 - ASTM D610) would call for spot repairs to be undertaken. A full coating removal is also not required if the defect or damage has not penetrated to bare metal and primer is not affected. In all cases, before undertaking spot repairs, it is prudent to determine that the surrounding coating that appears good is indeed sound enough. This can be assessed by visual tests, random adhesions tests (like pull-off tests that measure cohesion/adhesion) and tests to ensure corrosion has not propagated (under-film corrosion). Another important factor is whether the coating manufacturer confirms that the coating can be applied over surfaces cleaned as per SSPC-SP 3. For example, zinc silicates applied as part of heat- resistant coatings systems are not usually recommended over power tool-cleaned surfaces. Owners may also decide based on case specific factors to use power tool cleaning for an entire surface, based on it being the only method viable, safe (HSE), within maintenance cycles and budgetary allocation at that point in time.