Quality is a defining factor that determines success or failure. When we remove consistent quality from an equation (which can be in the form of organization, craftsmanship, and care), any given goal is doomed to fail. Inconsistency has an impact upon one’s results and that is why maintaining constant quality on the surface preparations of metal is of the utmost importance.
Why is Surface Preparation Important before Coating?
The surface of steel is prepared for the coating and is defined as one of the primary steps in preventing unwanted corrosion. The overall process of surface preparation and coating is conducted with the intention of protecting steel from corrosion. So why is surface preparation so important to factor in before coating steel?
It is important because coating systems literally depend upon the quality of the surface preparation. A badly prepared surface reduces the effectiveness of the coating and its anti-corrosive components. When a metal surface has not been prepared or to a low quality, the coating will struggle to bond to the steel. This is because the surface will still contain a presence of undesirable contaminants such as grease, oil, and more. Quality surface preparation will remove these contaminants and create a suitable plane for the anti-corrosive coating to hold onto.
Nonexistent or low-quality surface preparations = Unreliable anti-corrosive coating
Difference of consistency between UHP and Abrasive?
UHP (Ultra High Pressure) Water Blasting and Abrasive Blasting are two different methods of surface preparation commonly used before the coating of steel. Both UHP and abrasive blasting function with the purpose of removing paint, rust, and other contaminants from the steel surface. The difference is found, however, in how and to what degree this is achieved. So what is the difference in consistency exactly?
UHP Water Blasting
UHP will only remove surface contaminants such as paint, scale, and rust; it does not change the profile of the steel in any way. Once the surface contaminants have been blasted away successfully the consistency is known as a “near white finish”. A near white finish in appearance seems to be dull brown or gray in coloring. This blasting method cleans steel back to the original plane it was before painting or contamination occurred (which is an SSPC/ NACE WJ1 standard or ISO & SIS SA3 standard finish). Reverting back to the original surface consistency allows for fabrication work to continue (such as welding) with greater ease. UHP is capable of producing 4 finish consistencies which include:
- WJ-4 Light Cleaning: the surface is free of visible contaminants and all loose material; tough to remove material is permitted.
- WJ-3 Thorough Cleaning: random rust stains and thin deposits of tough to remove material is permitted on 33% of each 9 in2 of surface.
- WJ-2 Very Thorough Cleaning: same finish as WJ-3, but the percentage of contaminants is limited to 5% on each 9 in2of surface.
- WJ-2 Bare Substrate: the surface is completely free of all contaminant material; discoloration of the surface may be present.
Once surface preparation has been completed something known as “flash rusting” may become visible on the surface of the steel. Flash rusting is defined as the appearance of rust, post surface preparation, and it appears to be light – dark brown in its color. If light brown, the flash rust is considered acceptable and not of concern, while if dark brown, it would mean that the surface preparation was not conducted with efficiency and reblasting may be required.
Unlike UHP, abrasive blasting surface preparation will in fact change the surface consistency of the steel. Abrasive blasting will remove most of the unwanted contaminants, but also a small percentage of the steel itself. Because of this deep penetration, the removal of surface contaminants is thorough, and when properly executed it is almost guaranteed that the bond between the surface and later applied coating will be effective. In appearance and consistency, abrasive blasting generally produces a smooth, pale finish which is perfect for the application of anti-corrosive coatings. 5 common abrasive blasting finish consistencies include:
- SP 7/No. 4 Brush-off Blast Cleaning: All visible oil, grease, loose paint and other surface contaminants are removed.
- SP 14/No. 8 Industrial Blast Cleaning: Visible oil, grease, and loosely adherent materials are removed. Difficult to remove material may remain on 10% of each 9 in2 sections of surface,
- SP 6/No. 3 Commercial Blast Cleaning: Visible oil and grease are removed. Only streaks and stains of other contaminants are allowed on 33% of each 9 in2 sections of surface.
- SP 10/No. 2 Near-White Blast Cleaning: Same as SP 6/No. 3, but streaks and stains are allowed only on 5% of each section 9 in2 of surface.
- SP 5/No. 1 White Metal Blast Cleaning: No visible contaminants are permitted anywhere on the surface. The surface is completely clean.
Abrasive blasting can be slightly problematic with its performance as it can be quite aggressive in its finish consistency. The steel product worked upon can change in dimensions or even shape when blasted uncarefully. Utilizing wet abrasives is a great remedy and prevention method.
The Main Differences?
|UHP Water Blasting||Abrasive Blasting|
|Removes surface contaminants and rust only||Removes both surface contaminants and surface layer of steel|
|Does not create surface profile for coating||Does create surface profile for coating|
Coarser finish as some contaminants may be present
Smoother finish as contaminants are completely removed
Automation vs Manual blasting
Automated blasting equipment (such as blasting robots) and manual blasting methods (such as handheld sandblasters) are highly productive when used under the right circumstances. A flash torch used to illuminate a factory would be impractical as would a setup of industrial lighting for reading in the living room! Let us explore the pros and cons and the ideal applications for each method.
Automated Blasting: Pros
Automated blasting systems within the industry are becoming more prevalent as we discover more practical assets from their application. Blasting robots, in particular, can be extremely useful for minimizing human error, increasing consistency, and improving efficiency. A blasting robot can be remotely controlled by an operator or programmed to complete a task completely or semi-independent. The benefit of removing a manual operator from the scenario is that the margin for human error is minimized by a big threshold, this also improves the overall blasting consistency, control and efficiency (of economy and time).
Automated Blasting: Cons
This method can be costly, however, in return, it is to be expected to see an improvement in quality and speed. Automated systems are also limited with their functionality on different surfaces. For example, magnetic crawler blasting robots can only operate on ferrous materials because non-ferrous materials don’t have enough magnetic properties.
Manual Blasting: Pros
The three major benefits associated with manual blasting are lower costs of operation and improved accessibility. Manual blasting is generally much cheaper than automated methods due to the minimization of modern technology, which often requires much more training to utilize and can be expensive to replace components. Accessibility is a significant benefit manual blasting has over automated. A manual blaster allows one to blast nearly anywhere, no matter the surface plane, location, or size. If the operator can point his finger in the direction of the steel, he is likely to have accessibility and can prepare its surface easily.
Manual Blasting: Cons
Consistency is the downfall of using manual blasting as a surface preparer. This is largely connected to human error; an operator is bound to make mistakes or become tired throughout the day. This guaranteed aspect of human nature means that inconsistencies in the finish are inevitable.
Constant quality throughout steel surface preparations proves to be important for the livelihood of the asset being blasted and for the application of anti-corrosive coating systems. Much thought is required when considering the method of blasting and surface quality as without care, problems are bound to reoccur on the surface of your steel in the future.
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