AirSprayTech Academy Certificate Program
Secondary Containment Coating Systems for Industrial Contractors
Article 09 of 24
Surface Preparation of Steel in Containment Service
Steel containment surfaces must be cleaned, repaired, profiled, inspected,
and protected before contamination or corrosion can compromise the lining system.
The quality of this work directly affects adhesion, film continuity, and service life.
Learning Objectives
After completing this article, you should be able to:
- Explain why steel preparation requirements depend on the specified lining system and service environment.
- Distinguish surface cleanliness from surface profile.
- Recognize fabrication defects that can prevent proper lining coverage.
- Understand the importance of removing oil, salts, dust, rust, and spent abrasive.
- Document whether prepared steel satisfies the project specification before coating begins.
Steel Preparation Is Part of the Lining System
Secondary containment linings may be exposed to chemicals, standing liquids,
washdown water, weather, temperature changes, abrasion, and repeated wet-dry
cycles. A coating that performs well on properly prepared steel may fail rapidly
when applied over contamination, loose scale, inadequate profile, sharp edges,
or deteriorated welds.
Surface preparation is therefore not a general cleaning operation. It is a
controlled process performed to meet the written project specification and the
lining manufacturer's current requirements. The required standard, profile,
environmental limits, and acceptance criteria must be established before work begins.
Recommended Preparation Sequence
- Review the lining specification, product data sheets, drawings, and acceptance requirements.
- Inspect the steel for contamination, corrosion, section loss, fabrication defects, and existing coatings.
- Remove oil, grease, and other surface contaminants before abrasive blasting.
- Complete required welding, grinding, edge treatment, and steel repairs.
- Test for soluble salts when required and reduce contamination to the specified limit.
- Abrasive blast the steel to the specified degree of cleanliness and surface profile.
- Remove dust, spent abrasive, and loose debris.
- Inspect cleanliness, profile, dust, salts, fabrication details, and environmental conditions.
- Correct deficiencies and obtain documented acceptance.
- Apply primer or lining before the prepared surface deteriorates or becomes contaminated.
Remove Oil and Grease Before Blasting
Abrasive blasting should not be expected to remove oil or grease. Blasting an
oily surface can spread contamination and drive it into the surface profile.
The steel may look clean while retaining material capable of interfering with
lining adhesion.
Pre-cleaning should follow the project-specified procedure, commonly based on
AMPP/SSPC-SP 1, Solvent Cleaning. Suitable detergents, emulsifying cleaners,
approved solvents, steam cleaning, or other methods may be used as permitted.
Cleaning residue must also be removed. Contractors should verify that the
selected cleaning product is compatible with both the substrate and lining system.
Selecting the Required Blast-Cleaning Standard
The project specification determines the required degree of surface cleanliness.
Two standards frequently encountered in demanding lining work are:
-
AMPP/SSPC-SP 5/NACE No. 1, White Metal Blast Cleaning:
Used when the specification requires the complete removal of visible oil,
grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products,
and other visible foreign matter.
-
AMPP/SSPC-SP 10/NACE No. 2, Near-White Metal Blast Cleaning:
Allows only limited staining as defined by the standard while requiring
the removal of visible contaminants from the surface.
White metal preparation is more stringent, but it should not automatically be
substituted for near-white preparation or vice versa. The correct standard depends
on the specified lining, exposure, warranty requirements, and owner's criteria.
Visual reference photographs or comparators can assist evaluation, but they do not
replace the written standard. Lighting, viewing distance, access, and the condition
of the original steel can affect visual judgment.
Cleanliness and Surface Profile Are Different
Surface cleanliness describes what has been removed from the steel. Surface profile
describes the microscopic peak-and-valley pattern produced by abrasive blasting.
A surface can meet the visual cleanliness requirement and still have an unacceptable
profile.
A profile that is too shallow may not provide adequate mechanical attachment.
A profile that is too deep can consume excessive material and leave profile peaks
with insufficient coating coverage. The required profile range must come from the
lining manufacturer or project specification.
Profile may be evaluated with replica tape, depth micrometers, comparators, or
another approved method. Measurements should be taken at representative locations
and recorded. Contractors must use the method and frequency required by the project.
Abrasive Selection and Equipment Condition
Abrasive type, size, hardness, shape, and operating pressure influence production
rate, cleanliness, and profile. The abrasive must be capable of producing the
specified result without introducing harmful contamination.
- Confirm that the abrasive is clean, dry, and approved for the work.
- Protect stored abrasive from rain, ground moisture, oil, and debris.
- Inspect hoses, couplings, nozzles, controls, and moisture separators.
- Provide clean, dry compressed air of sufficient volume and pressure.
- Conduct compressed-air cleanliness testing when required by the specification.
- Control and collect blasting debris in accordance with site and environmental requirements.
Welds, Edges, and Fabrication Defects
Coatings tend to pull away from sharp edges and irregular welds as they cure.
This can leave a reduced dry-film thickness at the locations most likely to
experience early corrosion. Before final blasting, the contractor should inspect
the steel for conditions that cannot be corrected by coating alone.
- Sharp edges, corners, and cut plate
- Weld spatter and welding residue
- Rough, porous, or discontinuous welds
- Undercut, pinholes, pits, slivers, and laminations
- Skip welds, crevices, and inaccessible voids
- Bolts, brackets, stiffeners, and other difficult details
Edges should be rounded or treated to the degree required by the specification.
Weld spatter and sharp projections should be removed. Defective welding must be
referred to the owner, fabricator, or responsible engineer for repair.
Stripe coating is normally used on welds, edges, bolts, corners, and other complex
details to help achieve the required film thickness and continuity. Stripe-coat
material, timing, sequence, and application method must follow the approved lining procedure.
Coatings Cannot Restore Structurally Unsound Steel
Deep pitting, perforations, severe corrosion, inadequate welds, and significant
section loss are not coating defects. A lining may provide chemical and corrosion
protection, but it does not restore the original structural capacity of damaged steel.
Questionable conditions must be documented and referred to the owner or qualified
engineer before coating proceeds.
Soluble-Salt Contamination
Chlorides, sulfates, nitrates, and other soluble salts may remain on steel even
after it appears visually clean. If they are trapped beneath a lining, they can
attract moisture and contribute to osmotic blistering, corrosion, and loss of adhesion.
The specification should identify the test method, permissible contamination level,
test locations, frequency, and corrective procedure. Testing should concentrate on
areas likely to have been exposed to chemicals, coastal salts, process solutions,
repeated wetting, or previous leakage.
If contamination exceeds the specified limit, the surface may require fresh-water
washing, pressure washing, approved chemical cleaning, repeated blasting, or another
documented procedure. The surface must then be retested. Blasting alone may expose
or redistribute salts without reducing them to an acceptable level.
Dust and Abrasive Removal
Dust reduces direct contact between the lining and prepared steel. Spent abrasive
can collect on horizontal surfaces, behind angles, inside pits, around bolts, and
along containment transitions.
Use clean vacuuming, brushing, or dry oil-free compressed air as permitted by the
specification. Inspect from multiple directions under adequate lighting. Dust testing
should be performed when required, and the accepted cleanliness level should be recorded.
Preventing Flash Rust and Recontamination
Freshly blasted steel is highly reactive. It can rust rapidly when exposed to
condensation, high humidity, rain, contaminated air, or water. It can also be
recontaminated by dirty gloves, footwear, hoses, fuel exhaust, leaking equipment,
airborne process residue, or adjacent work.
Air temperature, steel temperature, relative humidity, and dew point should be
measured at the intervals required by the specification. The steel must remain
within the lining manufacturer's permitted temperature and dew-point separation.
If the specified degree of cleanliness is lost before coating, the affected surface
must be prepared again. A schedule deadline is not justification for coating over
rust, condensation, dust, or contamination.
Prepared-Surface Acceptance Checklist
- Correct preparation standard and approved procedure confirmed
- Oil, grease, chemicals, and cleaning residue removed
- Required repairs and fabrication corrections completed
- Specified blast-cleanliness level achieved
- Surface profile measured and within the required range
- Soluble-salt results within the specified limit
- Dust and spent abrasive removed
- Welds, edges, pits, bolts, and difficult details accepted
- No visible flash rust, condensation, or recontamination present
- Environmental readings acceptable for coating application
- Inspection results, test locations, instruments, and corrective work documented
Safety and Exposure Control
Abrasive blasting creates high noise levels, airborne dust, flying debris, visibility
restrictions, high-pressure hazards, and potential exposure to toxic substances.
Existing coatings may contain lead, chromates, or other regulated materials.
The contractor must evaluate the hazards, establish containment and ventilation,
use properly selected respiratory and personal protective equipment, control access,
and comply with applicable OSHA and environmental requirements. Confined-space
requirements apply whenever the work location meets the regulatory definition,
regardless of how briefly personnel enter it.
Technical References
Use the editions and requirements identified by the contract documents. Confirm
current designations before incorporating any standard into a proposal or work plan.
Key Takeaways
- Follow the specified preparation standard instead of relying on appearance alone.
- Remove oil and grease before abrasive blasting.
- Verify both visual cleanliness and surface profile.
- Treat welds, edges, pits, and fabrication defects as separate preparation concerns.
- Test for soluble salts when required by the exposure or specification.
- Protect blasted steel from condensation, flash rust, dust, and recontamination.
- Do not use a lining to conceal structurally deficient steel.
- Document acceptance before primer or lining application begins.
Professional responsibility:
This article provides foundational educational information and is not a substitute
for the project specification, engineering direction, regulatory requirements, or
the lining manufacturer's current instructions. Always review applicable technical
data sheets, safety data sheets, surface-preparation standards, inspection procedures,
and site safety requirements before beginning work. Obtain written clarification when
requirements conflict.
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