Why Coatings Fail

There are numerous reasons why coatings fail including poor surface preparation, surface contaminants, inadequately mixed paint, incorrect environmental conditions during paint application, poorly developed coating specification etc.

What most coating failures have in common is adhesion problems and the paint is no longer protecting the surface.

Rusty surfaces should not be painted!

Is this true?

Is there such a thing as ‘clean rust’?

Abrasive Blast Cleaning

Abrasive blast cleaning is considered the best form of surface preparation for coating application as the surface has been cleaned of paint and rust and the surface has a surface profile or ‘etched’ surface.

Specific standards have existed in the coatings industry for many years that show different levels of surface preparation standards with the use of abrasive blast cleaning.

There is ample evidence to support the fact that high levels of surface preparation and correct coating application will assist in a long service life of a coating system.

Abrasive Blast Cleaning

Abrasive blast cleaning is a process of propelling a material against the substrate at high pressure to remove the rust or paint.

There are various types of abrasive used including iron silicate (copper slag) which is one of the main expendable types of abrasive used for site works.

The abrasive breaks down very quickly and creates a lot of dust and debris.

This is an illustration of an abrasive blast cleaning operative working on a steel structure. It is not always feasible or practicable to get suitable extraction in every location.

Steel Abrasives

If steel abrasives are used, the waste is minimal as the abrasive can be used many times over. There is less dust and it is considerably easier for the blast cleaning operative to see the structure that is being cleaned.

This process is usually performed at fixed locations such as paint shops, fabrication yards etc.

Unless the abrasive can be recycled and the location is dry the only practical option is expendable abrasive such as iron silicate.

Environmental Considerations

In recent years there has been environmental pressure to reduce the extent of abrasive blast cleaning as a method of surface preparation and find alternative methods.

While asset owners have an option with alternatives on methods of surface preparation the balance is maintaining costs and not reducing the service life of the painted structure.

Alternatives

There are a number of alternatives to traditional dry abrasive blast cleaning as a method of surface preparation including;

• Wet abrasive blast cleaning (abrasive and water)
• Blasting with the use of specialist abrasives
• Disking, grinding or needle gunning
• High Pressure Water Jetting

Wet Abrasive Blast Cleaning

Wet blasting is basically the same as dry abrasive blast cleaning, although water is mixed with the abrasive in the blast pot, blast hose or at the nozzle.

A surface profile is produced as you are using abrasives and soluble salts will be removed as you are also using fresh water.

Wet Abrasive Blast Cleaning

This photograph was taken on a steel structure 90 minutes after wet abrasive blast cleaning was completed.

Although the surface is similar to a surface that has been high pressure water jetted, the wet abrasives have collected on the horizontal surfaces and the surrounding areas are still damp.

Collection of the wet abrasive can be very problematic.

Blasting with the use of Specialist Abrasives

It is possible to use specialist abrasives with abrasive blast cleaning that minimize any environmental impact.

These abrasives include:

• Sponge
• Plastics
• Ice

Although these abrasives are suibable for smaller works they are not suitable for large scale projects were high production is required.

Pressures vary with the type of abrasive. For example plastics break down very quickly and are used at very low pressures so production is very low compared to say iron silicate as an abrasive.

Mechanical forms of Preparation

Mechanical forma of preparation include disking, grinding and needle gunning.

This form of preparation is predominately used for small repairs. The process is labour intensive and does not fully clean the surface.

Generally a low coating life is offered when using mechanically cleaning compared to abrasive blast cleaning.

High Pressure Water Jetting

As you can see from previous slides the use of high pressure water jetting can be an alternative to abrasive blast cleaning in many instances, however the prepared surface will either be wet, damp or a surface that has oxidized to some extent.

This oxidation is however clean rust.

So what is clean rust?

Rust

Rust is formed when the surface (usually metal) reacts with the environment.

Steel is used in abundance in the construction industry and if it is not protected and maintained the steel will react with the atmosphere and corrode.

Oxide layers expand and can be contaminated by atmospheric particles such as soluble salts.

This form of rust is unstable and unsuitable for over painting.

High Pressure Water Jetting

If a steel surface is water jetted at very high pressures, rust, coatings and contaminants such as the soluble salts will be removed and you will be left with a cleaned, albeit wet surface.

As the surface dries the surface flash rusts.

Initially the substrate will be warm created by the energy of the water on the surface so drying can be relatively quick.

High Pressure Water Washing

This is an example of a hull of a ship that has been high pressure water jetted. The photograph was taken 60 minutes after the water jetting was completed in that location.

The speed of drying and flash rusting will depend upon the general environment including air and steel temperature, relative humidity and air movement.

Flash Rusting Standards

As it is inevitable that the degree of flash rusting will vary, Standards Organisations and Protective Coatings Societies have developed visual guides with different levels of flash rusting.

Theses visual guides should be in the coating contract and agreed by the contractor and coating supplier.

These visual guides will be discussed in a later training unit.

Flash Rusting Boundaries

When is too much flash rusting unacceptable. Obviously we have the visual guides, however the flash rusting should not produce a powdery surface which eventually appears when the surface dries.

Depending upon the environment this can be in a number of hours or over a number of days.

If a powdery surface is produced and the coating is thereafter applied, adhesion problems are inevitable.

The powdery substance is loose and will fall away from the substrate taking the coating with it.

This is an extremely important point when using high pressure water jetting and protective coatings.

Flash Rusting

If the extent of flash rusting exceeds the specified requirements you have the following options.

1. Steel brush to remove loose rust debris
2. Hand abrade
3. High pressure water wash
4. High pressure water jetting

Items 1 and 2 will require a full dust down and clean, however the surface will remain dry.

Items 3 and 4 will be quicker, however you will be wetting the surface again and drying the surface will be required.

Wet Substrates

There are coating suppliers who have developed coatings that can be applied over damp surfaces.

Two type products are available;

1. Solvent free epoxies specially formulated to be applied by brush and therefore restricted to small areas (spot repairs)
2. Solvent based primers formulated to go onto damp surfaces. If this type of coating is over applied then it could fail prematurely.

Always comply with the paint suppliers data sheets and recommendations

Is it Wet or is it Damp

If a coating supplier recommends that the coating can be applied over damp but not wet substrates this usually creates a problem on sites as the interpretation is not clearly defined.

One dictionary description of damp is ‘slightly wet’

By far the safest method is to allow the surface to dry and apply coatings to the dry surface.

Paint Application

As there will be no requirement to apply a blast primer (which is usually applied to hold the blast cleaned substrate), stripe coating is usually recommended before the first spray coat as illustrated.

On larger areas the stripe coating squad can work ahead of the spray painters.

Depending upon the environment the spray application may have to be applied with stripe coating applied when practical.

Paint Suppliers

Many of the paint suppliers have coatings which can be applied to substrates which have been high pressure water jetted and allowed to dry.

Coating based on surface tolerant epoxies for example have been know to give good results.

Once the primer is applied the system can be top coated like any traditional coating system.

For general atmospheric environments a typical coating system will be x1 or 2 coats of epoxy mastic (high solids) at 200 microns (8 mil) and a urethane or modified acrylic topcoat at 50 microns (2 mil)

Shipyards

There are many shipyards globally which do not allow the use of abrasive blast cleaning and paint suppliers have developed coatings to be applied onto high pressure water jetted surfaces.

This photograph illustrates an epoxy aluminium primer application over a surface that has been high pressure water jetted.

Providing the surface does not deteriorate, the performance will equal some of the blast cleaning standards previously used in shipyards as a method of surface preparation.

Environmental Conditions

Even though the surface has been allowed to oxidize (ginger) this does not mean that coatings can be applied under any environmental condition.

The paint must be applied in accordance with the suppliers recommendations. These will be found in the product data sheets.

Future Developments

The use of high pressure water jetting as a method of surface preparation will increase.

More and more asset owners are looking at alternatives to abrasive blast cleaning without the reduction in coating performance.

The paint suppliers are doing more research and development with coatings on water jetted surfaces.

The water jetting equipment costs and practicalities for use will be the major drawback with this process, however there is expected to be an increase in automated and robotic water jetting surface preparation equipment.

Rust!

There are various levels of rust and then there are various levels of contaminated rust.

Water jetting will remove surface rust and clean a surface free from contaminants such as soluble salts.

Is this clean rust?

can you paint this surface?

Water Jetting

When you Water jet a rusted surface the initial standard or surface preparation is equivalent to an abrasive blast cleaned surface as illustrated in this photograph.

The energy of the water is transferred to the steel surface which heats up.

This heat instantly dries the surface and oxidation occurs.

This oxidation is clean rust and suitable for coating with various products.

The oxidation should not be allows to build up into a dust or a powder as this create adhesion problems with the coating system.

Coatings should be applied as soon as practical.

Mechanically Prepared Surfaces

Over the years coating suppliers have developed coatings that can be applied onto mechanically prepared surfaces.

These coatings are called surface tolerant and include epoxy mastics.

The same coatings can be applied to water jetted and dried substrates and should (in theory) perform better as the surface will not be polished and free from surface contaminants.

Once the first coating has been applied the surface has been sealed and the remaining coatings can be applied as normal.

Coating Application

Were practical the first coat onto a water jetted surface should preferably be applied by brush as this will assist with the surface wetting and promote adhesion.

On general maintenance, brush application is often used.

Spray application is still used, such as the sides of ships and vessels. This process is typically airless spray. Brush application would be both time consuming and expensive for large areas.

Summary

We have reviewed the use of manual and automated water jetting equipment used as a method of surface preparation.

We have also reviewed the surface finish with regards to the flash rusting.

As the use of water jetting increases in the coatings industry the applicator should have an understanding of the process and the implications of applying coatings which do not comply with the paint manufacturers recommendations.