Getting surface prep right is non-negotiable. Here’s some of our advice for tackling surface preparation, even when it’s complicated.
Surface preparation drives coating success. Failures often originate from improper or incomplete surface preparation. In this newsletter, we’ll discuss surface prep materials, preparatory cleaning, achieving the proper anchor profile, guidance on standards, and what to do when abrasive blasting (or grit blasting) is challenging in the application environment.
Why Surface Prep Matters
Coating failure analyses can trace the incident back to the root cause of that failure. Often, the root cause is a lack of proper surface preparation, which can result from an improper anchor profile or the presence of residual contaminants, such as dirt, dust, and salts.
Proper surface preparation recommendations always depend on the substrate and the coating material. Concrete substrate preparation is not the same as steel substrate preparation. The proper surface profile for thin film coatings is different than a high-build epoxy.
Manufacturers should be providing clear specifications for the surface preparation for their coatings. At Advanced FRP, we work directly with applicators to formulate the best surface prep procedure for the repair at hand.
Preparatory Cleaning
Before applying the coating, the substrate must be free of both visible and invisible contaminants. These may include:
Oil, grease, dirt
Loose or broken mill scale
Rust
Chemical salts, like chloride and sulfates
Old coatings
Surface condensation
Pre-cleaning is necessary for surfaces that may be contaminated with grease or oil. Blast cleaning will not remove these contaminants. In fact, using blasting, you may spread the oil and grease, contaminating the abrasive-blast media.
In the case of converting a No. 6 Fuel Oil tank to a Biodiesel tank, cleaning the surfaces to be free of oil was extremely challenging. Over three months, three separate attempts to pressure wash and clean the tank could not adequately remove areas of residual oil from tight corners. In this case, the coating system that was ultimately applied used an oil-tolerant primer, per Advanced FRP Systems’ recommendation. This oil-tolerant primer, FRP 201 OT Primer, was specifically designed for this type of application. According to testing performed during the project, the primer consistently provided over 2,500 psi adhesion, even on steel surfaces with heavy oil contamination.
In a particularly challenging surface prep scenario, residual oil was difficult to remove from the tight corners present in the tank.
Inspection
It’s recommended practice to inspect cleanliness three times:
Before any surface preparation activities
After prep, before coating.
Between coats in multi‑coat systems.
To inspect, use visual guides for consistency. Capture photos of the initial condition and the cleaned results. Coating manufacturers often recommend at least a NACE No.2/SSPC-SP-10 “Near-White Metal Blast Cleaning” standard for immersion grade coatings, if not a NACE No.1/SSPC-SP-5 “White Metal Blast Cleaning”.
The Danger of Chlorides and Other Ions
The presence of chlorides or other ions on metal surfaces can accelerate osmotic blistering, a type of coating failure that occurs when water permeates through a coating (osmosis). The table below shows commonly used standards for soluble ion concentrates, including chloride concentrations.
Table 1. Soluble Salt Maximum Concentrations*
*From Chlor-Rid International’s Non-Visual, Soluble Salt Contamination of Steel Substrates
Osmotic blistering most likely caused by the presence of chlorides on the substrate from improper cleaning before coating.
Proper Anchor Profile
An anchor profile, also called a surface profile or anchor pattern, describes the “peaks and valleys” created after grit blasting a surface. Creating an anchor profile on the surface of the substrate increases the available surface area and improves adhesion through mechanical bonding.
Mechanical bonding refers to a type of adhesion where two materials are held together primarily by physical interlocking rather than chemical or molecular forces. Velcro is a great example of mechanical bonding on a visible scale. Coatings use a similar means of bonding.
The depth of the anchor profile should be measured in accordance with the ASTM D4417-11 standard. A typical minimum profile depth will be 2 or 3 mils, but this can vary based on the coating being used. A thinner coating may require a shallower surface profile, while a thicker coating may need more depth. Care should be taken to adhere to manufacturer-provided standards; too low of a profile may cause premature coating failures, such as peeling or blistering.
Delamination of coating most likely due to insufficient or improper surface prep.
What’s New: SSPC‑SP 18 (2020)
Published on December 30, 2020, the SSPC-SP 18 standard defines Thorough Spot and Sweep Blast Cleaning for previously coated steel surfaces using dry abrasive blast cleaning. The procedures outlined in this standard are designed to subject the entire surface being prepared to abrasive impact. Coating that cannot withstand the blasting process is removed, while the remaining coating is adequately prepared for the application of another layer of coating.
Implications
Overcoating intact systems with localized failures is common in maintenance. SP‑18 standardizes acceptance for proper surface preparation in these cases. The industry benefits from less ambiguity and more clarity. The new standard also enables inspectors to verify compliance, ultimately providing asset owners with peace of mind regarding the durability of the coating.
Special Cases: When You Cannot Blast
Grit blasting may be restricted in certain situations, like where ignition risk is high or in a confined space. There are alternatives to grit blasting, though it is the best way to get the proper surface profile for adhesion.
When diagnosing your repair, we work with the end user and the applicator to design surface preparation procedures and understand the root cause of any elevated risk. If you are avoiding grit blasting, we discuss why that may be and what systems may be in play that pose unnecessary risk.
If grit blasting is determined to be impossible, then we can consider alternatives, like bristle blasting.
In this repair on the coal pipe elbows in a power generation facility, the surfaces were prepped using bristle blasting on the extrados of the elbows. The internal maintenance team was trained by Advanced FRP Systems in this situation, and successfully installed the system on three elbows within a two-day mill outage. While the mill was taken off-line, the plant itself was still online throughout the repair.
Caption: Bristle blasting was an alternative to grit blasting that was used in this coal power generation facility. Coal dust can pose an ignition risk, so bristle blasting was the chosen alternative.
Additional Reading
We have several resources linked above in the article and available on our Resources page for more information about how to set your repair up for success early on in the process. If you have any additional questions, please reach out for a free consultation.
The Surface Preparation Playbook
Getting surface prep right is non-negotiable. Here’s some of our advice for tackling surface preparation, even when it’s complicated.
Surface preparation drives coating success. Failures often originate from improper or incomplete surface preparation. In this newsletter, we’ll discuss surface prep materials, preparatory cleaning, achieving the proper anchor profile, guidance on standards, and what to do when abrasive blasting (or grit blasting) is challenging in the application environment.
Why Surface Prep Matters
Coating failure analyses can trace the incident back to the root cause of that failure. Often, the root cause is a lack of proper surface preparation, which can result from an improper anchor profile or the presence of residual contaminants, such as dirt, dust, and salts.
Proper surface preparation recommendations always depend on the substrate and the coating material. Concrete substrate preparation is not the same as steel substrate preparation. The proper surface profile for thin film coatings is different than a high-build epoxy.
Manufacturers should be providing clear specifications for the surface preparation for their coatings. At Advanced FRP, we work directly with applicators to formulate the best surface prep procedure for the repair at hand.
Preparatory Cleaning
Before applying the coating, the substrate must be free of both visible and invisible contaminants. These may include:
Pre-cleaning is necessary for surfaces that may be contaminated with grease or oil. Blast cleaning will not remove these contaminants. In fact, using blasting, you may spread the oil and grease, contaminating the abrasive-blast media.
In the case of converting a No. 6 Fuel Oil tank to a Biodiesel tank, cleaning the surfaces to be free of oil was extremely challenging. Over three months, three separate attempts to pressure wash and clean the tank could not adequately remove areas of residual oil from tight corners. In this case, the coating system that was ultimately applied used an oil-tolerant primer, per Advanced FRP Systems’ recommendation. This oil-tolerant primer, FRP 201 OT Primer, was specifically designed for this type of application. According to testing performed during the project, the primer consistently provided over 2,500 psi adhesion, even on steel surfaces with heavy oil contamination.
Inspection
It’s recommended practice to inspect cleanliness three times:
To inspect, use visual guides for consistency. Capture photos of the initial condition and the cleaned results. Coating manufacturers often recommend at least a NACE No.2/SSPC-SP-10 “Near-White Metal Blast Cleaning” standard for immersion grade coatings, if not a NACE No.1/SSPC-SP-5 “White Metal Blast Cleaning”.
The Danger of Chlorides and Other Ions
The presence of chlorides or other ions on metal surfaces can accelerate osmotic blistering, a type of coating failure that occurs when water permeates through a coating (osmosis). The table below shows commonly used standards for soluble ion concentrates, including chloride concentrations.
Table 1. Soluble Salt Maximum Concentrations*
*From Chlor-Rid International’s Non-Visual, Soluble Salt Contamination of Steel Substrates
Our informative article on common causes for coating failures in water immersion conditionsdives deeper into surface prep and cleaning in underwater or water immersion applications.
Proper Anchor Profile
An anchor profile, also called a surface profile or anchor pattern, describes the “peaks and valleys” created after grit blasting a surface. Creating an anchor profile on the surface of the substrate increases the available surface area and improves adhesion through mechanical bonding.
Mechanical bonding refers to a type of adhesion where two materials are held together primarily by physical interlocking rather than chemical or molecular forces. Velcro is a great example of mechanical bonding on a visible scale. Coatings use a similar means of bonding.
The depth of the anchor profile should be measured in accordance with the ASTM D4417-11 standard. A typical minimum profile depth will be 2 or 3 mils, but this can vary based on the coating being used. A thinner coating may require a shallower surface profile, while a thicker coating may need more depth. Care should be taken to adhere to manufacturer-provided standards; too low of a profile may cause premature coating failures, such as peeling or blistering.
What’s New: SSPC‑SP 18 (2020)
Published on December 30, 2020, the SSPC-SP 18 standard defines Thorough Spot and Sweep Blast Cleaning for previously coated steel surfaces using dry abrasive blast cleaning. The procedures outlined in this standard are designed to subject the entire surface being prepared to abrasive impact. Coating that cannot withstand the blasting process is removed, while the remaining coating is adequately prepared for the application of another layer of coating.
Implications
Overcoating intact systems with localized failures is common in maintenance. SP‑18 standardizes acceptance for proper surface preparation in these cases. The industry benefits from less ambiguity and more clarity. The new standard also enables inspectors to verify compliance, ultimately providing asset owners with peace of mind regarding the durability of the coating.
Special Cases: When You Cannot Blast
Grit blasting may be restricted in certain situations, like where ignition risk is high or in a confined space. There are alternatives to grit blasting, though it is the best way to get the proper surface profile for adhesion.
When diagnosing your repair, we work with the end user and the applicator to design surface preparation procedures and understand the root cause of any elevated risk. If you are avoiding grit blasting, we discuss why that may be and what systems may be in play that pose unnecessary risk.
If grit blasting is determined to be impossible, then we can consider alternatives, like bristle blasting.
In this repair on the coal pipe elbows in a power generation facility, the surfaces were prepped using bristle blasting on the extrados of the elbows. The internal maintenance team was trained by Advanced FRP Systems in this situation, and successfully installed the system on three elbows within a two-day mill outage. While the mill was taken off-line, the plant itself was still online throughout the repair.
Caption: Bristle blasting was an alternative to grit blasting that was used in this coal power generation facility. Coal dust can pose an ignition risk, so bristle blasting was the chosen alternative.
Additional Reading
We have several resources linked above in the article and available on our Resources page for more information about how to set your repair up for success early on in the process. If you have any additional questions, please reach out for a free consultation.