Corrosion protecting properties of coatings depend on the constituent parts of the material and on how well the coating provides complete coverage of the surface.

If the coating is not spread adequately a burst air bubble in the coating film can lead to a so-called pore through which corrosive influences can reach the surface to be protected.

Modern pore test instruments make use of the conducting properties of the substrate, by measuring electrical current that can be made to flow through existing pores in the coating material, using the measuring probe of the test instrument.