Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers



Last updated: July 15, 2019

What Does Overcoating Mean?

Overcoating refers to an additional protective coating on surfaces. Overcoating on curved surfaces and unprepared surfaces causes mud-cracking. Overcoating may also cause an accumulation of paint and sagging. Overcoating is commonly applied to bridge painting projects to control corrosion.

Overcoating provides many benefits when the coating systems have been selected judiciously and employed properly with full-time inspection. In selecting overcoating, corrosion control considerations should prevail over coating film attributes.


Corrosionpedia Explains Overcoating

Overcoating is an extra protective coating over existing coating. Before applying overcoating, the following need to be considered:

  • Check adhesion and film integrity – whether good adhesion to the substrate and between coats exists, without film defects like cracking or blistering.
  • Degree of rusting – confirm that rusting is less than 10%.
  • Cleaning of overcoat systems – use power washing with detergent and spot power tool to clean surface, dirt, oil, etc.
  • Geographic location – consider exposure conditions and relative humidity.
  • Risk factor – determine the degree of risk associated with the existing coating system delaminating between coats, or from the substrate after application of an overcoat system, taking into account freeze/thaw cycling and total film thickness of the existing system.
  • Coating materials selection.

Nearly 50% of overcoating systems use epoxies, as well as thicker phosphate. These provide better corrosion protection when combined with an overcoat of paint. Paint overcoating can be applied instantly after the corrosion coating with no loss of corrosion protection. Overcoating can also be used immediately after the thermal spray coating.

Overcoating can cause solvent entrapment when overcoating times are not adhered to or when painting is directly carried out under high temperatures. The additional heat causes the paint to set too quickly and trap solvents. This is usually common when applying darker colors.


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