AirSprayTech Academy Certificate Program
Secondary Containment Coating Systems for Industrial Contractors
Article 10 of 24
Primers, Sealers, and Bonding Layers
The first material applied to a prepared substrate can determine how well the entire
containment system bonds, wets the surface, controls porosity, and resists pinholes,
outgassing, corrosion, and chemical exposure.
Learning Objectives
After completing this article, you should be able to:
- Explain the functions of primers, sealers, and bonding layers.
- Recognize why concrete and steel may require different primer technologies.
- Understand how porosity, moisture, temperature, and surface condition affect primer selection.
- Identify common primer-application errors that can compromise a containment lining.
- Verify primer coverage, cure, recoat condition, and compatibility before applying the next layer.
A Primer Is a Functional Part of the System
A primer should never be treated as an inexpensive layer placed beneath the
“important” coating. In a properly designed containment system, every layer performs
a function. The primer is the material responsible for establishing initial contact
between the prepared substrate and the succeeding lining, membrane, mortar, or laminate.
Depending on the system, a primer may wet the substrate, penetrate surface pores,
improve adhesion, reduce concrete outgassing, seal absorbent areas, temporarily
protect blasted steel, or create a chemically compatible surface for the next coat.
Some containment systems require a separate primer. Others use a reduced or specially
formulated first coat. Certain systems may be designed for direct application to a
specific prepared substrate. The contractor must follow the tested system and the
manufacturer’s written instructions rather than assuming that every lining requires
the same type of primer.
Primer, Sealer, or Bonding Layer?
These terms are sometimes used interchangeably, but they can describe different
functions. The product data sheet and system specification should define the role
of each material.
-
Primer:
A first coat formulated to promote adhesion or provide another required
interface between the substrate and the coating system.
-
Sealer:
A material used primarily to penetrate or reduce the porosity of concrete,
masonry, or another absorbent surface.
-
Bonding layer:
An intermediate material used to create adhesion between surfaces or system
components that may not bond reliably without it.
-
Tie coat:
A compatible intermediate coating used to connect dissimilar coating
technologies or provide a suitable surface for a succeeding coat.
-
Scratch coat:
A thin resin-rich layer or filled material worked into an irregular surface
to fill minor voids and provide a more uniform base for subsequent layers.
The Primer Does Not Correct Poor Surface Preparation
A high-quality primer cannot compensate for laitance, weak concrete, oil
contamination, soluble salts, dust, loose rust, mill scale, condensation, or an
incorrect surface profile. Applying primer over an unacceptable substrate only
conceals the condition until the lining fails.
The prepared substrate must satisfy the project specification before primer
application begins. Once the primer is applied, important evidence of contamination,
cracking, corrosion, and inadequate preparation may no longer be visible.
Priming Concrete
Concrete presents different challenges from steel. It is porous, variable, and
capable of containing or transmitting moisture and air. Even after mechanical
preparation, one area may absorb primer rapidly while another remains comparatively
dense.
A concrete primer may be expected to:
- Wet and penetrate the prepared surface.
- Bind residual surface particles that remain within acceptable limits.
- Reduce uneven absorption of the next layer.
- Fill small pores and reduce pinhole formation.
- Improve adhesion between concrete and the lining system.
- Create a suitable base for mortar, membrane, laminate, or high-build coating.
Concrete that absorbs the primer immediately may require additional material or
another application, but only when permitted by the manufacturer. The objective is
not to create an unnecessarily thick glossy film. Excessive primer can trap air,
solvent, or unreacted material and can interfere with the bond of succeeding layers.
Concrete Outgassing
Outgassing occurs when air or vapor within concrete moves toward the surface and
escapes through freshly applied material. The escaping gas can produce pinholes,
bubbles, craters, or small openings that remain in the cured coating.
A common risk period occurs when the substrate is warming. Air within the concrete
expands as temperature rises and can move outward through the wet primer. Applying
during stable or falling substrate temperatures may reduce this risk when the product
instructions and project conditions permit.
The contractor should not assume that a primer will eliminate every outgassing
problem. Highly porous concrete, bugholes, cracks, moisture movement, rapid temperature
change, and improper application can still produce discontinuities.
When outgassing is observed, stop and determine the cause. Repeatedly rolling over
a material that has begun to cure can make the surface worse. The corrective procedure
must follow the manufacturer’s instructions and may require sanding, filling, repriming,
or removal of defective material.
A Primer Is Not Automatically a Moisture-Mitigation System
Some primers tolerate limited substrate moisture. That does not mean they are
designed to control moisture-vapor transmission, hydrostatic pressure, or chronic
water intrusion.
Where moisture is present, the contractor must distinguish among a conventional
primer, a damp-surface primer, and a tested moisture-mitigation system. Concrete
moisture results must be evaluated against the written limitations of the complete
system. Product substitution based only on general descriptions such as
“moisture tolerant” can result in blistering and delamination.
Priming Steel
A steel primer must wet the abrasive-blasted profile and provide a compatible
interface for the specified lining. It may also provide temporary protection against
corrosion while the next layer is being installed.
The primer must be applied before the steel develops flash rust or becomes
contaminated. Dust, spent abrasive, condensation, fingerprints, dirty footwear,
exhaust, oil, and airborne process residue can compromise a freshly blasted surface.
The primer must reach and wet the valleys of the blast profile. Excessively thick
application can bridge the profile, create solvent retention, or exceed the
manufacturer’s permitted film thickness. An application that is too thin may leave
profile peaks insufficiently protected.
Stripe coating may be required at welds, edges, bolts, pits, corners, and other
complex details. The specification should identify whether stripe coating occurs
before or after the general primer coat and whether it must be brushed, rolled,
or applied by another method.
Primer Selection Questions
Before approving or applying a primer, determine:
- Is the primer part of the lining manufacturer’s tested system?
- Is it approved for concrete, steel, or both?
- Is it compatible with the chemical exposure and service temperature?
- Can it be applied at the measured substrate temperature?
- What moisture condition can the primer tolerate?
- What surface profile or concrete surface profile is required?
- What coverage rate and film thickness are specified?
- Does the product require induction time before application?
- What is its working time or pot life at the actual temperature?
- What are the minimum and maximum recoat times?
- What preparation is required if the recoat window is exceeded?
- Must aggregate be broadcast into the wet primer?
- Are special ventilation, ignition-control, or respiratory measures required?
Mixing and Material Control
Two-component primers depend on accurate proportioning and complete mixing.
Estimating proportions by eye, splitting kits without suitable measuring procedures,
or scraping partially cured material from the side of a container can produce soft,
uncured, or chemically weak areas.
Follow the manufacturer’s requirements for:
- Material conditioning and storage temperature
- Component ratio
- Mixing speed and mixing time
- Container shape and mixer type
- Induction or sweat-in time
- Pot life and working time
- Permitted thinner and maximum thinning amount
- Batch size and application equipment
Pot life normally becomes shorter as material temperature increases. Material can
become unsuitable before it becomes visibly solid. Do not add solvent to restore
apparent workability unless the manufacturer expressly authorizes the procedure.
Application Methods
Primers may be applied by brush, roller, squeegee, conventional spray, airless spray,
or another approved method. The selected method must place the material uniformly
while working it into the surface texture and difficult details.
On porous concrete, the applicator may need to work the primer into the prepared
surface instead of merely depositing it on top. On blasted steel, the application
must wet the anchor profile without creating excessive thickness, runs, sags,
dry spray, or missed areas.
Spray application may improve production, but it does not eliminate the need for
backrolling, brushing, stripe coating, or detail work when required by the procedure.
Corners, penetrations, curbs, welds, bases, and transitions should be inspected
separately from broad open surfaces.
Coverage Rate and Film Thickness
Theoretical coverage is based on a smooth surface and does not account for substrate
porosity, blast profile, surface texture, transfer loss, waste, or material remaining
in containers and equipment. Actual coverage can vary substantially.
Material usage should be compared with the area coated. Unusually high consumption
can indicate excessive absorption, roughness, overapplication, leakage, waste, or an
incorrect area calculation. Unusually low consumption can indicate thin application,
missed areas, inaccurate measurements, or excessive thinning.
Wet-film thickness may be measured when the product and application permit.
Dry-film measurements on rough steel require a method appropriate for the substrate
and coating. Concrete primer thickness may be difficult to measure directly because
of absorption and surface variation, making coverage records and careful visual
inspection especially important.
Recoat Windows and Intercoat Adhesion
A minimum recoat time allows the primer to develop enough cure to receive the next
layer. A maximum recoat time identifies how long the next material may be applied
without additional preparation. Both limits can change with temperature, humidity,
ventilation, film thickness, and product chemistry.
Coating too early may disturb the primer, trap solvent, or interfere with cure.
Coating too late may reduce chemical adhesion and require cleaning, abrasion,
or application of another bonding material.
The crew should record when each area was primed, not merely when the day’s work
began or ended. Large containment areas may need to be divided into identifiable
zones so each section remains within its allowable recoat window.
Amine Blush and Surface Contamination
Some amine-cured epoxy materials can develop a surface film commonly called amine
blush. Its occurrence depends on formulation and environmental conditions. The film
may interfere with adhesion of the next coat.
Do not assume that solvent wiping will remove the condition. Follow the coating
manufacturer’s procedure, which may require washing with clean water and an approved
cleaner, thorough rinsing, drying, and mechanical abrasion.
The primed surface must also be protected from dust, rain, condensation, insects,
oil, overspray, construction debris, and traffic. A primer that was acceptable when
applied may no longer be acceptable after uncontrolled exposure.
Primer Acceptance Checklist
- Specified product, color, batch numbers, and shelf life verified
- Substrate preparation inspected and accepted before priming
- Substrate moisture and environmental conditions acceptable
- Components conditioned, proportioned, and mixed correctly
- Required induction time observed
- Material used within its permitted pot life and working time
- Specified coverage and film thickness achieved
- Porous or highly absorbent areas properly treated
- Welds, edges, corners, pits, penetrations, and transitions covered
- No pinholes, bubbles, craters, runs, sags, dry spray, or missed areas
- Primer sufficiently cured for the next operation
- Maximum recoat time has not been exceeded
- Surface remains clean, dry, and free of contamination
- Inspection results and corrective work documented
Common Primer Failures
-
Pinholes:
Often associated with concrete porosity, outgassing, incorrect application
timing, inadequate wetting, or excessive working of material as it begins to cure.
-
Soft or uncured areas:
May result from incorrect proportioning, incomplete mixing, unsuitable
temperature, contamination, or use beyond pot life.
-
Peeling or delamination:
May indicate contamination, insufficient surface preparation, moisture,
incompatibility, or application outside the recoat window.
-
Bubbles or blisters:
May be associated with outgassing, moisture movement, trapped air, solvent
retention, osmotic activity, or application under unsuitable conditions.
-
Rusting through the primer:
May indicate insufficient film thickness, excessive profile, contamination,
delayed coating, condensation, or premature weather exposure.
-
Intercoat adhesion loss:
May result from contamination, amine blush, excessive cure, an exceeded
recoat window, or incompatible materials.
Required Documentation
Primer records should identify:
- Project area and location
- Product name, color, batch number, and expiration date
- Quantity mixed and quantity applied
- Mixing, induction, start, and completion times
- Air temperature, substrate temperature, relative humidity, and dew point
- Surface condition and substrate moisture results when applicable
- Application method and equipment used
- Coverage rate and available thickness measurements
- Observed defects and corrective actions
- Time the next layer was applied
- Inspector or responsible applicator acceptance
Technical References
Use the editions and requirements identified in the contract documents.
Standards do not replace the lining manufacturer’s system-specific instructions.
Key Takeaways
- The primer is a functional component of the containment system.
- A primer cannot correct contamination or inadequate surface preparation.
- Concrete primers must address porosity, absorption, and outgassing.
- Steel primers must wet the blast profile without leaving peaks inadequately protected.
- A conventional primer is not automatically a moisture-mitigation system.
- Mixing accuracy, pot life, film thickness, and recoat timing directly affect performance.
- Every primed area should be inspected and accepted before it is covered.
- Good documentation protects the contractor, owner, and completed system.
Professional responsibility:
This article provides foundational educational information and is not a substitute
for the project specification, engineering direction, regulatory requirements,
or the coating manufacturer’s current written instructions. Always review the
complete system specification, technical data sheets, safety data sheets,
surface-preparation requirements, application instructions, and site-safety
procedures before beginning work. Obtain written clarification when requirements conflict.
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