The protection of iron and steel is a serious problem, and many millions of rands are spent annually to prevent or limit corrosion and on replacement of corroded metal.
The important factors in the protection of iron and steel by painting are:
- The thoroughness of the preparatory work before painting.
- The type of primer used.
- The thickness of the paint system.
As far as preparatory work is concerned, it has been shown that the effective life of a paint coating of perfectly clean steel, free from rust and scales, may be up to five times longer than that of the same coating applied to imperfectly cleaned steel. The traditional manual methods of preparing new and old steelwork for painting by chipping, scraping and wirebrushing are far from ideal, as they cannot remove lightly-adherent scale nor deal effectively with rust on pitted or rough surfaces. Alternative methods such as pickling, grit blasting, flame cleaning and chemical derusting are either not practical on erected steel, or may be considered too expensive. It should be realised that the higher initial cost would be more than offset by a reduction in maintenance costs by the longer life of the paint coatings.
For general use on iron and steel, primers incorporating rust-inhibitive pigments must be used, for example, zinc phosphate, metallic zinc, etc., in various binders such as the conventional linseed oil, or alkyd resins, or epoxy resins, etc. Too often cheap and inferior primers are used containing less than the minimum quantities of anti-corrosive pigments to be effective. Such paints do more harm than good as they must first be removed before a systematic and effective painting operation can be carried out.
Steelwork must be primed as soon as possible after it has been prepared, whether on site or at the manufacturer’s works, as even a few hours exposure in a damp or contaminated atmosphere will nullify the value of thorough preparation. The thickness of the overall paint is important and at least three full coats must be applied.
Clean down to remove dirt, grease, oil, etc. Sand or grit blast to remove all rust and millscale. If sandblasting is not economically viable, chip, scrape or wirebrush to remove rust and loose scale back to clean metal. Where it is not possible by manual methods to remove rust from pitted or rough surfaces, apply a proprietory rust convertor such as Dekro Hydromet (RCC) or a similar product in accordance with the manufacturer’s instruction. Cleaned and/or treated surfaces must be primed immediately.
If in poor condition, remove by scraping, burning or with Dekro Super Paint Stripper. Then proceed as for new substrate. If in good condition, clean down thoroughly with a suitable solvent to remove dirt, grease, oil, etc. Sand glossy surfaces with suitable abrasive and dust off.