Index of Surface Conditions

Surface Preperation

This section includes pictorial examples of some typical conditions, from bare steel through to failed coating system, likely to be encountered before any surface preparation is carried out. Also illustrated are some typical problem areas which need to be appropriately treated prior to or during full surface preparation.

Various photographs are included which show mill scale on unprepared steel at various stages of degradation.

Whenever grinding is used for removal of defects, professional advice should be sought to ensure that the integrity of the structure or vessel is not compromised. Strict limits are usually applied to the thickness of metal which can be removed by grinding e.g. with high pressure pipework.

Burrs           

Bolt holes and other cut areas should be thoroughly checked for burrs and rough edges, which should be removed by grinding or other suitable means.

Contamination on surface

Oil, grease and other foreign contamination must be removed by solvent cleaning or the use of an alternative suitable biodegradable cleaner prior to full surface preparation.

Edges

All edges should be radiused to provide a smooth round surface to which the paint can adhere to provide good protection.

Feathering

Where repairs are carried out to an already painted surface, the edge of the old paint should be feathered to a sound, well adhered thin edge using an appropriate means of abrasion, prior to overcoating.

Laminations, Surface Shelling or Hackles

The steel surface should be checked for laminations both before and after surface preparation. Any laminations found must be removed by grinding before application of any coating system. The use of an Ultrasonic thickness gauge may be necessary to determine the extent of delamination.

Mill Scale

All mill scale should be removed by appropriate cleaning techniques, usually abrasive blast cleaning, prior to application of a coating system.

Surface Porosity

All surface porosities should be filled with an appropriate filler to provide a smooth surface.

Slag

All welding slag should be removed by mechanical means so that the underlying weld may be inspected and suitably prepared. Coatings should not be applied over welding slag.

Spatter

All weld spatter should be removed by mechanical means.

Undercuts

Undercuts should not be excessive or rough.

Welds

Welds should be continuous and free from sharp projections. See also the section on Welding Defects.

Corrosion Trap

Description: Poor design resulting in a well where water can collect.

Comment: This problem should be eliminated wherever possible at the design stage, by including appropriate drainage holes or modifying the design to prevent areas where water or corrosive fluids can collect.

Poor Design

Description: Poor design resulting in areas that are innaccessible for surface preparation or paint application.

Comment: This problem should be eliminated wherever possible at the design stage by taking into consideration protection from corrosion and accessibility for preparation and paint application.

Edges - Flame Cut

Description: Sharp rough edges resulting from flame cutting.

Comment: All such rough cut edges should be ground smooth and any sharp edges radiused. A radius of 2 mm is generally recommended for good painting practice.

Edges - Sharp

Description: Untreated sharp edges on steel sections.

Comment: Remove all sharp edges prior to surface preparation. A radius of 2 mm is generally recommended for good painting practice. Apply a stripe coat over all edges to ensure adequate coverage by the coating system.

Mill Scale

Several photographs are included to show the effect of outdoor exposure on untreated rolled steel stock. The extent of degradation of the surface will depend on the type of exposure conditions, whether Rural Unpolluted, Industrial Polluted, or Marine. The examples included are purely to illustrate the various types of surface conditions which might be encountered.

Millscale - "A" Quality Steel

Description: A close view of “A” quality steel with virtually intact blue mill scale.

Millscale - "A" Quality Steel

Description: Steel section where intact blue millscale is present with only  light superficial rust in some places.

Millscale - "B" Quality Steel

Description: Showing some breakdown of the millscale after weathering and formation of corrosion products.

Millscale - "B" Quality Steel

Description: An example of steel with a “B” quality surface condition. The steel sections have been exposed to a rural unpolluted external atmosphere for a period of 2-3 months. The corrosion process has caused some degradation of the mill scale and a light film of rust has formed on the steel surfaces.

Millscale - "C" Quality Steel

Description: The majority of the millscale has weathered away leaving corrosion products on the surface.

Millscale - "C" Quality Steel

Description: An example of a “C” quality surface condition. The steel sections have been exposed to a rural unpolluted external atmosphere for a period of 4 - 6 months. The corrosion process has caused total degradation of the mill scale and an overall film of rust has formed on the steel surfaces.

Oil Contamination

Description: Partial oil contamination of the surface with an oil film.

Comment: All oil and grease must be removed from the surface using solvent or biodegradable detergent degreasing prior to full surface preparation. Unless this is properly carried out, a residual oil film will be transferred to the surface during dry blast cleaning, which will prevent good adhesion of the coating system.

Skip Weld

Description: Non-continuous weld or skip weld.

Probable Causes: Used where a full weld is unnecessary for constructional purposes.

Repair: Ideally a continuous weld should be used. Where this is impractical, the gaps should be cleaned and filled with a suitable mastic to prevent bridging by the coating.

Comment: Unless the gaps are properly treated these areas will serve as initiation points for corrosion.

Slag

Description: Weld slag which has not been removed from the surface.

Comment: All welding slag should be removed by mechanical means so that the underlying weld may be inspected and suitably prepared. Coatings should not be applied over welding slag.