Technical Guide:

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

Initial surface treatment for steel plates

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

• Oil or grease shall be removed by wiping or scrubbing the steel with clean rags or brushes wetted on solvent. Deposits firmly adhering to the steel shall be removed by scrapping and then cleared by solvent.
• Corrosive salts such as chlorides and sulphates, etc on the steel surface shall be rinsed with fresh water. Water and moisture shall be removed by wiping the steel with dry rags or by drying the steel with hot forced 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 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, air blower etc.

Secondary Surface Treatment for Fabricated Steel and Application of repair Paint

Defective areas with damage and rust owing to gas cutting, welding, and stress relieving measures must be cleaned by blasting or with power tool cleaning. Degreasing ad 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 fiber or wire brush or a combination of both.
• Deposited oil and grease must be removed using solvent.
• Use a blast cleaner or power tool to remove weld flux slags, weld metal spatters, weld flux fume deposits, rusted and damaged paint films in the welded areas.
• Use preferably a vacuum cleaner to remove dust, sand residue and other contaminants.

General guidelines for pretreatment methods

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 work in order to get high performance and durability of coated materials. Some factors that will influence the selection of pretreatment method are summarized as follows:

• The physical and chemical cleanliness of the surface
• Surface conditions
• Surface profile
• Characteristics of the paint.
• Safety aspect
• Environmental compulsions
• The kind of tools which are available
• The kind of previous treatments

When the type of pretreatment and paint system have to be decided, the large sums of money involved should always be kept in mind.

Following are the methods of pretreatment which can be used to remove under rusted paint:

Comparison of different International Standards of Surface Preparation

* The steel Structure Painting Council Specification
The National Association of Corrosion engineers Specification
The Swedish Standard
The British standard specification

Aluminum/Tin/Copper/Brass and other Non-ferrous metals:

• 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 by chemical cleaning.
• 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.

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.