Factors to consider in surface preparation and painting
A deficient preparation of the surface to paint followed by a good painting system can lead to worse results than if you use low quality products on a surface that has been previously prepared correctly.
Many factors influence the type of prior surface preparation and the choice of the painting system to use, which will mark the quality of the finish paint.
- Nature of the material to paint: the type of preparation and the time and costs involved will vary depending on whether you are going to treat a metal, plastic or another type of part.
- Previous condition of the surface: you should assess the condition of the surface to treat, since it will influence the choice of preparation method.
- Existence of rust or old paint coats: this is especially relevant if the part is inside or outside, and which is the dominant climate. The level of oxidation can vary (surface, light, medium and strong oxidation).
- Mechanical conditions that the part may suffer: if the part or surface is exposed to continuous friction, vibration or mechanical impact, this should be considered for the pre-treatment to be carried out.
- The size of the surface to prepare and its geometry
- The cost of the process and application.
- The colour and brightness you want to achieve.
Types of surface preparation for painting
The presence of fats and oils is quite frequent in structural materials and must be completely removed before starting painting and repainting.
The most common procedures are cleaning with solvent-based or even water-based degreasers.
The most usual method for cleaning with solvents is using a moistened cloth. This cleaning can also be carried out by spraying, aspersion and even immersion in the vapour phase.
The solvent used should have the ability to remove grease and oils, it should be sufficiently volatile and have low toxicity. The cloths used must be renewed frequently to avoid greasing them.
The degreasing process can also include washing with water and detergents or disinfectants at very high pressure, to remove salts, rust, grease or old paint.
Manual and mechanical cleaning
Manual cleaning is the process used to prepare surfaces by brushing, scraping, grinding or sanding of the metal to remove rust, loose mill scale or old paint.
This is a slower and more laborious method used to clean and prepare the surface for painting, which does not fully remove impurities, but is useful when other processes cannot be used because the parts are hard to reach or because of the cost. The brushes usually used for this process are made of steel wire or barbed wire.
Based on the UNE EN ISO 12944-4 standard, the following levels of preparation by manual scraping and brushing are considered:
- Level ST 2: This level of preparation removes mill scale, rust and weakly adhering paint coatings and foreign matter. The surface must show a metallic appearance.
- Grade ST 3: mill scale, rust and weakly adhering paint coatings and foreign matter are removed However, the surface must be treated more intensively than for grade ST 2, to provide a metallic shine from the substrate. The surface should have a pronounced metallic appearance.
This type of preparation allows for the surface to be blasted with small particles of abrasive materials thrown at high speed by compressed air through a flexible hose.
The sanding material used can be steel blasting, synthetic abrasives, etc.
Without a doubt, this is the most effective procedure for cleaning and preparing the surface since it removes rust, scale, welding residues, oils, etc., while achieving an excellent roughness that favours adhesion of the coats of paint.
It is important to take into account that the metal subjected to blasting has a very active surface that is easily oxidised by environmental humidity or atmospheric pollutants and must therefore be coated with paint immediately.
According to the SIS-055900 specification (surface preparation regulations), the blasting preparation levels are divided into:
- Grade Sa 1: light blasting. Mill scale, rust and weakly adhering paint coatings and foreign matter are removed.
- Grade Sa 2: careful blasting. Mill scale, rust and paint coatings and foreign matter are removed. Any residual contamination must be strongly adhered to. The surface should be greyish in colour.
- Grade Sa 2½: very careful blasting. Mill scale, rust and paint coatings and foreign matter are removed. Any traces of contamination should appear only as small circles or stripes. The surface should have an almost white metal appearance.
Grade Sa 3: demanding blasting. Mill scale, rust and paint coatings and foreign matter are removed. The surface should have a uniform white metal appearance.
With this treatment, the surface of the steel part is transformed into a thin coat of iron phosphate to improve both resistance to corrosion and adhesion of paint coats.
Before phosphating, the surface must be free of rust, fats and oils.