What does each layer do?

PositionFunctionALKA starting pointProject control
Prepared carbon steelSound foundation for adhesion and corrosion control.Surface cleaned and profiled to the selected primer’s approved requirements.Contamination, blast cleanliness, profile, welds, edges and ambient conditions.
PrimerProtects and bonds at the steel interface.ALKA-404 zinc-rich epoxy in the published ALKA-9930 build.Primer identity, zinc-rich qualification where specified, contact with prepared steel and overcoating condition.
MIO epoxy intermediateAdds barrier film above the primer.ALKA-413 micaceous iron oxide epoxy.Specified film build, edge coverage, intercoat condition and compatible next layer.
Polyurethane finishDelivers the exposed colour and weathering appearance.ALKA-421 two-component acrylic polyurethane finish.Colour and gloss approval, UV exposure, film build, recoat and cure schedule.

MIO is a lamellar pigment: in an appropriately formulated and applied intermediate coat, its plate-like particles contribute to a barrier layer. The public ALKA-413 Revision 4 document describes that role, but it does not establish a universal service life, chemical-resistance rating or individual-coat thickness for every system. Film continuity and the approved total build still matter.

Choose the system by atmospheric exposure and finish

ALKA’s ALKA-9930 Premium External combines ALKA-404 zinc-rich epoxy primer, ALKA-413 MIO epoxy and ALKA-421 polyurethane finish. It is the published three-layer starting point where exposed structural steel needs both a corrosion-control foundation and a specified exterior appearance. ALKA-9933 High-Barrier Atmospheric instead emphasises a selected primer plus ALKA-413 barrier; a compatible finish can be added when colour and gloss retention matter. The precise primer and finished build must be defined before it is treated as the same system as ALKA-9930.

A simpler visible-steel route is ALKA-9934 Industrial Finish, which describes a selected primer plus ALKA-421 without a separate MIO intermediate. It may fit a different exposure and maintenance brief; it is not interchangeable with a three-coat barrier system on name or appearance alone. The ALKA-9934 system page identifies ALKA-416 as an epoxy zinc-phosphate primer, while the ALKA-416 product page identifies a single-component zinc-rich coating. Obtain written confirmation of that product code and compatible primer before using any ALKA-416 schedule.

Define the asset’s conditions before choosing among these routes. Is it an interior beam, weather-exposed frame, coastal structure, pipe rack or tank exterior? How often is it wet, and will chlorides or industrial deposits remain on the surface? Is appearance visible to occupants, or is access for future repainting difficult? ISO 12944-2 provides the environment-classification framework, while ISO 12944-5 guides selection of protective paint systems. The category and durability level for a project require a supported specification; they cannot be assigned to ALKA-9930 simply from the generic three-coat sequence.

Preparation and details decide whether the build works

Before blasting, remove oil, grease and process residues by an approved cleaning method. Inspect the steel for old paint, mill scale, corrosion, pits, weld defects, sharp edges and inaccessible crevices. Specify an actual cleanliness grade and surface profile matched to the primer, not just “clean steel.” ISO 8501-1 covers visual rust and preparation grades; ISO 8503-1 covers profile comparators. Where salt contamination is relevant, name the test and project acceptance limit; ISO 8502-6 covers extraction of water-soluble contaminants.

Prepare welds, cut edges and other imperfections to the required grade. ISO 8501-3:2025 addresses those details. Plan stripe coats for edges, welds, bolts and complex geometry where the approved specification calls for them, then inspect the full-coat coverage. A high nominal film thickness measured on an open face says little about a missed edge, a crevice or a contaminated weld.

Coat the prepared steel within the specified conditions. Record substrate and air temperature, relative humidity, dew point, visible moisture and any weather change. After the zinc-rich primer, inspect for damage, contamination and the permitted overcoating condition. Confirm whether ALKA’s approved procedure requires a sealer or mist coat before applying ALKA-413. Apply ALKA-413 and ALKA-421 within their validated intervals; if a window is exceeded, obtain the required cleaning or abrasion procedure. Do not assume that a dry-looking epoxy is ready to receive polyurethane.

Specify and inspect the complete film

State individual coat and total dry-film thickness (DFT), permitted range, surface preparation, application method, stripe-coat sequence, environmental limits, inspection frequency and repair procedure in the project schedule. The nominal total in an ALKA system page is a selection guide, not an instruction to apply that thickness in one coat. Its division among primer, MIO and polyurethane must be supported by the current technically approved product data.

Verify DFT with a calibrated method and agreed acceptance criteria. ISO 19840 describes measurement and acceptance on rough steel surfaces; ISO 12944-7 addresses execution and supervision in shop and field work. Record surface preparation, profile and contamination readings where specified, conditions, batches, coat measurements, repairs and final appearance. Inspect runs, dry spray, pinholes, misses and edge coverage before the next coat hides them.

Know the service boundary

These systems are described for atmospheric steel protection. MIO pigmentation does not automatically make ALKA-413 a tank lining; polyurethane weathering does not turn ALKA-9930 into a splash, tidal, immersed or chemical-containment system. Specify those duties separately with the exact fluid, concentration, temperature, exposure duration and required test evidence. Inspect drain traps and damaged areas periodically, remove deposits and plan compatible repairs before corrosion spreads beneath the coating.

Request an ALKA system review with drawings, steel condition, location, exposure, colour requirement, access and target maintenance plan. Confirm the approved ALKA-404/413/421 product versions, component instructions, coating schedule and SDS set before procurement and application.

Frequently asked questions

Can ALKA-413 be left as the visible finish?

ALKA-9933 describes an exposed MIO epoxy option where appearance is secondary. If exterior colour and gloss retention are important, discuss a compatible weathering finish such as the ALKA-421 layer in ALKA-9930. Confirm the complete specification rather than adding a topcoat informally.

Why use a polyurethane finish over MIO epoxy?

The epoxy intermediate provides barrier build; the polyurethane is the selected exposed finish for colour, gloss and weathering appearance. The two roles complement the primer, but performance still depends on surface preparation, compatible layers and the installed film.

Does three coats guarantee an ISO 12944 durability class?

No. An ISO category or durability claim needs a defined environment, preparation, paint build and relevant evidence. A product family or coat count alone is insufficient.

Technical references

Planning guidance only. Confirm the current technically approved ALKA TDS/SDS, compatibility, testing and application limits before specifying or installing a system.