Surface preparation
The importance of surface preparation is emphasized by SSPC in its Protective Coatings Inspector (PCI) Training Program, which is intended for those involved with coating projects. (For more on the Protective Coatings Inspector [PCI] Training Program, visit www.scribd.com/document/240673149/SSPC-Student-Intl-1.)
“Preparing the surface for subsequent application of the coating system is the most critical (and typically the most expensive) step in an industrial coatings project,” according to the SSPC PCI Training Program’s section on surface preparation. “In general terms, the better the surface preparation, the longer the life of the coating system.”
The program also advises ambient conditions such as surface and dewpoint temperature should be measured before initiating final surface preparation, recommending professionals verify “the temperature of the surfaces is at least 3 C (5 F) higher than the temperature of the dewpoint, to preclude airborne moisture from condensing on the surfaces.”
Additional information on surface temperatures, curing times, compatibility of coatings, and other technical data is contained in product data sheets (PDS), which are usually maintained on the coating manufacturer’s website or available from company representatives. In some situations, laboratory analysis from a coatings manufacturer may be required to identify the generic type of an existing coating system and assess the suitability of surface preparation methods for a specific project. The analysis can establish whether the old paint system is compatible with self-priming tie-coats and other coating candidates.
Prior to overcoating, existing paint systems must be dry and clean. This can be achieved by solvent cleaning in accordance with SSPC standards for solvent cleaning, hand-tool cleaning, and power-tool cleaning. (These standards include SSPC-SP1, Solvent Cleaning, SSPC-SP2 Hand-tool Cleaning, and SSPC-SP3, Power-tool Cleaning.)
Cleaning with wire brushes, scrapers, and other non-power tools can remove loosely adhering old paint and flaking mill scale. Rusty areas require power-tool cleaning with grinders and pneumatic chisels and spot priming before overcoating.
CSSBI recommends special attention and treatment be given to areas that may have already begun to corrode by removing all traces of white, black, or red rust, then applying a corrosion-resistant, zinc-rich primer.
“Low-pressure water cleaning (LPWC) or ‘pressure washing’ is often specified for overcoating projects,” according to the SSPC PCI Training Program. “LPWC can be very effective in removing dirt, chalking, bird droppings, and other contaminants from the surfaces, although mechanical agitation of the surface during LPWC is often required to ensure adequate removal.”
Overcoating requires the edges of existing coatings to be feathered using sandpaper or power-tool attachments to form a smooth transition prior to spot-priming. Failure to feather edges can result in poor adhesion and an undesirable appearance.
