Proper surface preparation is of great importance in obtaining the optimum film performance. So we have described the initial surface treatment for steel plates, secondary surface treatment for fabricated steel and the application of repair paint.

The following initial surface treatment is to be applied to steel plates.

• Oil or grease should be removed by wiping or scrubbing the steel with clean rags or brushes wetted with solvent. Deposits firmly adhering to the steel should be removed by scraping and clearing by solvent.
• Corrosive salts, such as chlorides and sulphates, on the steel surfaces should be rinsed with fresh water. Water and moisture should be removed by wiping the steel with dry rags or by drying the steel by forcing hot air.
• All mill scale, rust, rust-scale, paint marks or foreign matter shall be removed by grit or sand blasting to meet the ISO standard.
• Before the shop primer is applied, dust, sand residue, crushed steel shot or grit and all other contaminants must be removed from the surface using a vacuum cleaner or air blower.

Defective and damaged areas must be cleaned by blasting or with power tool cleaning. Degreasing and washing may also be necessary to clean the surface before the subsequent coats are applied. To do so, follow the steps below:

• Remove corrosive salts, chalks, marks, soil or other contaminants and foreign matter by brushing the steel with a stiff fibre or wire brush or a combination of both.
• Deposited oil and grease must be removed using solvents.
• Use a blast cleaner or a power tool to remove weld flux slags, weld metal spatters, weld flux fume deposits, rusted and damaged paint films in the welded areas.
• Use a vacuum cleaner to remove dust, sand residue and other contaminants.

The quality of surface preparation seriously affects the performance of paint films. Therefore, it is important to make the right choice of both, the method and the grade of surface preparation, before starting painting. Some factors that will influence the selection of the pre-treatment method are summarized as follows:

The physical and chemical cleanliness of the surface

• Surface conditions
• Surface profile
• Characteristics of the paint
• Safety aspects
• Environmental compulsions
• The kind of tools available
• The kind of previous treatments

When the type of pre-treatment and paint systems have to be decided, the large sums of money involved should always be kept in mind.  

*The Steel Structure Painting Council Specification
The National Association of Corrosion Engineers Specification
The Swedish Standard
The British Standard Specification

• Surface should be dry and clean.
• Any visible oil/grease should be removed.
• Cleaned surface should be abraded or sweep-blasted using low pressure and non-metallic abrasives, then primed with a coat of wash primer.  

Galvanized Steel

• Degreasing to remove any oil/grease.
• Any white zinc corrosion products should be removed by high-pressure, fresh water washing.
• Water washing is recommended to ensure removal of soluble zinc salts.

Stainless Steel

• Stainless steel surfaces do not require any specialized surface pretreatment prior to coating. These surfaces should be free from oil, grease, dirt, and other foreign materials.
• The development of a surface profile on stainless steel is highly recommended to ensure good coating adhesion.
• A profile depth of between 1.5 and 3.0 mills is suggested for most coating systems.

Concrete and Masonry Surfaces

New Concrete surface:

• Must be allowed to cure at least 30 days before coating.
• The moisture content of the concrete/masonry should be less than 6%.
• In case of large areas and for severe exposure conditions, the surface has to be prepared by light blasting. In less critical areas where blasting is not practical, wire brushing has to be adopted to remove laitance, followed by treating with dilute hydrochloric acid.
• Allow the surface to dry thoroughly before applying primer.

Old concrete surface:

• Remove the surface contaminants like grease, oil etc by solvent wiping or by 10% caustic solution.
• The surface should be preferably prepared by light blasting. In case blasting is not practical, etch the surface to get a good profile by treating with dilute HCl.
• Remove acid and contaminants by washing with water.
• Ensure that acid solution does not remain on the surface and joints.
• Allow the surface to dry thoroughly before applying primer.

Wood surfaces:

• Dirt/grease/oil should be removed by one or more chemical cleaning methods.
• Knots, nails, holes, cracks, etc should be filled with appropriate filler compound, scrape off loose adherent coating if any, and sand to even surface.
• Chalky surfaces should be washed cleanly and dried well before coating.