Protective coatings - painting steel

June 12th, 2020

Painting steel can pose particular problems for the professional applicator. The correct preparation and selection of the correct coating system is critically important if the applicator wishes to maximise performance and life expectancy. In worse case scenarios, the selection of an inappropriate coating system can result in complete failure!

Correct system selection requires the applicator to make two basic assessments:

1. The nature of the environment

Clearly the coatings that would be required for painting school railings in the countryside where the corrosive intensity of the environment is relatively low, would not be the same as those for painting an offshore structure where the corrosive intensity of the environment is extremely high.

Environments are graded in terms of corrosive intensity in accordance with ISO 9223 as follows:

- C1 Very Low: Inside heated buildings with clean atmosphere

- C2 Low: Areas with low levels of pollution and a dry climate. Rural areas and the inside of unheated buildings

- C3 Medium: Urban and light industrial atmospheres with moderate levels of sulphur dioxide pollution

- C4 High: Industrial areas and coastal areas with moderate salinity

- C5 Very High: Extreme industrial areas and coastal or offshore facilities with high salinity

2. The condition of the substrate and the degree of preparation that can be undertaken

If the substrate is previously painted and in good condition then it is likely to require a different system to a substrate that is for instance, previously unpainted and rusting very badly.

Additionally a substrate that can be prepared by abrasive blast cleaning to a very high degree of cleanliness is likely to warrant a different approach in terms of coating selection to a substrate that can only be wire brushed.

The following standards apply:

Abrasive blast cleaning in accordance with BS7079 Part A:

- SA3 White Metal

- SA2.5 Near White Metal

- SA2 Commercial Blast

Manual preparation in accordance with BS7079 Part C:

- ST3 Very Thorough Manual Preparation

- ST2 Thorough Manual Preparation

Most specifications will call for SA2.5 when abrasive blast cleaning is required and ST3 when manual preparation is required.