How Does Blasting and Painting Remediate Steel? Part Two of Two

By Daniel Green

Recently, we began detailing how blast and paint remedies corroded, aged and surface damaged steel. Let’s pick up that article where we left off.

Surface Profiling

Removal of existing surface coatings is clearly a blaster/painter’s objective. However, achieving the correct surface profile is equally important. By definition, this is the level of coarseness and pattern left behind after blasting. These factors go a long way towards the adhesion of new coatings as well as the aesthetics and kinaesthetic of the finished product.

Generally speaking, the more aggressive the media the harsher the profile. At the microscopic level, steel shot will leave an extremely coarse profile compared to acrylic or soda. Other factors that affect the surface are the air pressure, the distance of the wand from the surface, the angle at which you are blasting and how long you maintain contact with a given area. Like sandpaper, the more coarse the media and the longer you do it – the rougher the final product.

Too much profile will leave far too many peaks which promotes faster wear on those areas. Too little and the coatings won’t adhere. It has to be just right.

Types of Blasting

Further advantages can be had by altering the state of the media during the blasting process. Often, blasting is done with dry media. This will require dust suppression. But what if there were other upticks to changing the properties of media?

Wet blasting. The only known advantage is the ability to mix detergents in with the media, and by using hot water, surfaces can be degreased and blasted simultaneously.

Vapour blasting uses less water than wet blasting and is, therefore, kinder to high-tolerance surfaces such as mating flanges. The mist also keeps the dust down.

Micro-abrasive, also known as pencil blasting is where a minute volume of media passes through a very fine nozzle – usually 1mm². It is so fine that it can etch glass and eggshells.

Automated blasting is the blasting process without human intervention. This is perhaps where toxic gasses are regularly breaching safe levels.

Vacuum blasting removes blasting waste as you go. Recapturing blast material greatly reduces the inhalation risk of silicates – a common side effect of material blasting.

Safety

The primary risks in media blasting are:

• Dust. The presence of silicates in the blasting process has driven the industry to seek out new media. Unfortunately, even materials like stainless shot will still create dust. Water is frequently used as dust suppression, however, it does speed up the corrosion of freshly blasted ferrous metals.
• Abrasions. Media designed to strip scale from hardened surfaces escaping a blasting wand under high pressure will invariably lead to accidents. PPE has come a long way which had reduced the number of these incidents.
• Deafness. Millions of granules of media in swift flight and bouncing off tough surfaces is loud. Even with world-class hearing protection, there are many cases of permanent hearing damage for career blasters.
• Airborne contaminants. More of then than not the surface coating undergoing removal isn’t family-friendly. Reducing it to microscopic dust promotes inhalation, even with correct BA.

To help counter these risks, blasters wear specific PPE. The level of their protection depends on the scale of the work – the more industrial the more hardy the PPE.

• Communication. Modern blaster painters are in contact communication with the team outside the booth via a handsfree two-way radio headset.
• Blast suit. Specifically designed to withstand the immense pressure of hurtling media, a one-piece suit is worn while in the booth.
• Blast gloves. Made of the same materials or greater, these elbow-length gloves can withstand a direct hit from harsh UHP media such as steel shot.
• Fresh air supply. Drawn from outside the booth and pumped in via blast-proof hoses, ambient air has one job: keep the blaster alive.
• CO² sensor. This will sound an alarm when oxygen levels in the booth drop below critical.
• A/C. Inside all that PPE is extremely hot. Given that blasters can work for several hours at a time, it is vital to keep them cool.

Blasting Outcomes

Once the blasting process has been completed, all surfaces are back to base and the correct surface profile has been achieved then the steel must be etch primed immediately. Corrosion is the action of oxygen and so bare steel must be protected within hours, even minutes.

Once all substrates have been primed, a thorough inspection can begin. Corrosion is much like an iceberg: 90% of it lurks below the surface. Blasting reveals issues never before apparent. Everyday across the globe there are high-pressure vessels and pipes that are corroded to within microns of catastrophic failure. Wealthier nations with stronger safety standards that drive more frequent inspections and repairs usually catch these in time. Unfortunately poorer nations are subject to these outcomes which often result in the loss of life, precious resources and capital.

With a clear snapshot of any steel that doesn’t meet spec, repairs are made or replacement parts ordered. Painting can occur in stages or all at once depending on the components/project.

Surface Coatings

With thousands of variants of primers and paints available the correct materials for the project must be selected. This isn’t just in the case of coatings in relation to the substrate, it is in the case of coatings in relation to each other. Many materials are incompatible, resulting in poor or no adhesion, wrinkling or straight-out rejection.

If there is neglect in the appropriate choice of surface coatings, mother nature will reclaim things in a very short period of time. The work will have been for nought.

In Conclusion

Media blasting is a comparatively inexpensive process that allows a full overview of the state of components on a given piece of plant or equipment. It can be non-invasive, environmentally friendly, inert and toxin-free. It can etch steel, concrete, aluminium, timber, brick, copper and more. Media blasting is an extremely versatile procedure that can greatly enhance the functionality and aesthetics of your project.

For steel remediation enquiries contact Tom on

Glass Beads Get It Done.