The challenges of painting on residual rust.

The effectiveness of a paint system is determined by the surface preparation as much as by the quality of the coating. Even the finest paint will not offer complete protection when applied to a steel surface that still has rust present. Despite the availability of various surface preparation techniques, this issue has led to numerous challenges.

Most mechanical surface treatments, including scraping, hand brushing, and power cleaning, are unable to eliminate all the rust from surface cavities. The presence of residual rust in these areas promotes ongoing corrosion underneath subsequently applied paint films.

While blasting the surface to expose white metal creates an excellent base for paint, the freshly blasted surface is highly reactive. Upon exposure to atmospheric humidity, dew, or rain, rapid rusting occurs. Therefore, to preserve the benefits of blasting, the newly blasted surface must be painted without delay.

Despite its recognized advantages as a paint base, sandblasting is often disregarded due to factors such as cost, environmental concerns, setup inconvenience, dust and spark risks, size limitations for cleaning areas, and challenges in coordinating blasting and painting schedules.

Similarly, acid-based surface pre-treatment for corroded steel surfaces has shown limited effectiveness in addressing the issue. Rust removal under practical temperature, concentration, and application conditions proves extremely challenging.

In many cases, the presence of residual acids or salts trapped in rust pores has led to paint system failure through osmotic blistering. Even under optimal field conditions, traditional phosphoric acid-based pre-treatments fall short, failing to achieve full metal surface passivation.

These products do not provide complete passivation of the metal surface as the reaction with various layers of rust is not uniform and thorough.

To combat the issue of residual rust potentially promoting further corrosion and the unsatisfactory results of acid cleaning in the field, an ideal solution involves complete passivation of tightly bound residual rust to chemically inhibit its contribution to ongoing corrosion.

Learn more about our KELATE® solutions developed with this key principle in focus.