Metal Ferrous
General:
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 the 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, maybe 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 wire-brushing 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 realized 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 of 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.
Preparation:
New:
Clean down to remove dirt, grease, oil, etc. Sand or grit blast to remove all rust and mill scale. If sandblasting is not economically viable, chip, scrape or wire brush 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.
Previously painted:
If in poor condition, remove by scraping, burning, or with Dekro Super Paint Stripper. Then proceed as to 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.
First Coat: Suitable Dekro Primer
Second Coat: Suitable Top Coat
Third Coat: Suitable Top
Metal Galvanised
General:
A galvanized surface, especially when new, is particularly difficult to paint for the following reasons:
It is so smooth and does not provide a good key for most paints.
It is usually heavily pretreated by the steel manufacturer to prevent white rust on storage and this pre-treatment must be thoroughly removed before paint is applied.
Zinc as metal can react with the ingredients in some paint to form salts between the paint and the surface, thus further reducing adhesion. It was the practice in years gone by, either to allow the galvanized iron to weather to obtain a surface receptive for paint or to use mordants such as copper sulfate, vinegar, spirits of salts, etc. These methods are now obsolete having been proved technically unsound.
Preparation:
New:
Hose or rinse down with clean water to remove heavy dust, grit, or organic matter. Clean down with Dekro Galv-O-Clean Degreaser or Galv-Etch by working over the surface using a mop, brush, or hessian. Thoroughly rinse down with clean water to remove residues. Note whether the final rinsing water forms a continuous film over the surface. If it does, then cleaning is sufficient, if not, repeat the cleaning operation.
Previously painted:
If the existing paint is in poor condition, remove it by scraping, burning, or with Dekro Super Paint Stripper. Then proceed as to new work.
If the existing paint is in good condition wash down to remove dirt, grease, dust or other contaminants with a suitable concentrated detergent. Abrade glossy surfaces with a suitable abrasive and dust off.
First Coat: Suitable Dekro Primer
Second Coat: Suitable Top Coat
Third Coat: Suitable Top
Metal Non-Ferrous
General:
Aluminum and its alloys are widely used in building construction in the form of flat or ribbed sheets, extruded sections, cast components, etc. Whilst aluminum as a metal is largely self-protective because of its oxide layers, it may require paint for decoration or for protection under very severe corrosive conditions. Sheets and extruded sections have a smooth shiny surface and require pre-treatment to obtain a satisfactory key for paint, whilst cast aluminum requires no pre-treatment other than removal of dirt and grease.
Lead and brass are best left unpainted as paints do not adhere well to these surfaces. Copper must be well abraded before the application of paint.
Preparation:
New:
Clean to remove grease, dirt, oil, etc., by washing with a suitable concentrated detergent.
Previously painted:
If in poor condition, remove with Dekro Paint Stripper. Then proceed as to new work. If in good condition, clean down thoroughly with a suitable solvent to remove dirt, grease, oil, etc.
Sand glossy surfaces. Dust off.
First Coat: Suitable Dekro Primer
Second Coat: Suitable Top Coat
Third Coat: Suitable Top
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