Paint Defects 4 – Pinpoint Rusting

pinpointrustingSmall dots or points of rust on a primed or painted steel surface generally clustered in an area or groups of areas. Pinpoint rusting appears where the surface blast profile protrudes through the primer or finished coating and begins to corrode from environmental or corrosive exposure. It also appears at pinholes or areas where the coating is thin (often, but not always, shortly after the finished item is installed or exposed) or when steel is coated with a porous latex coating.

Paint damage showing at the paint surface as irregular shaped blisters. If the blisters burst then corrosion spots are clearly visible (e.g. brown rust on steel or white rust on aluminium).

CAUSES:

  • Low film thickness (more likely creating rust rashing), voids and holidays (more likely creating rust rashing), but also defects in the steel such as laminations and inclusions.
  • Too high a surface profile may cause penetration of peaks through a paint film and cause rust spotting.
  • May also occur from metallic contamination of a coated surface by grinding dust etc.

PREVENTION:

  • Ensure that an adequate thickness of a primer coat is applied to cover the surface profile.
  • Use a thicker coating system or a lower blast profile.
  • Protect coating from contamination with grinding dust etc.
  • Stone chips or scratches should be repaired immediately.
  • Ensure through cleaning of all metal surfaces with an appropriate metal cleaner. Remove all rust by sand blasting to produce a bright metal substrate. Tin all welded seams.
  • Sandblasted areas should be primed immediately after blasting to avoid flash rusting.
  • Seal and protect all cavities with the recommended products.

SOLUTIONS:

  • Depending on type and extent, abrade or blast clean surface and recoat.
  • Remove all foreign debris and abrade, clean damaged areas.
  • Apply spot repair or recoat overall.

REFERENCES:

  1. CoatingsMall, Paint Defects Advice, date of access: 11 July 2016 https://www.coatingsmall.com/cap_data_docs/documentum/Training%20(BASF_Training)/Demo/Paint_Defects.pdf
  2. The MPI Group, Coating Defects, date of access: 11 July 2016 http://www.mpigroup.co.uk/publishing/fitzs-atlas-of-coating-defects/translation/4-coating-defects/
  3. Mpiinfo, Pinpoint Rusting, date of access: 11 July 2016 http://mpiinfo.com/dsd/pinpoint-rusting/