Epoxy protects steel through adhesion and resistance to water and chemicals, but it chalks under UV; aliphatic polyurethane is the topcoat that keeps colour and gloss in sunlight; alkyd, simple and forgiving, suits mild environments and light maintenance. In practice you do not choose a single resin: you choose a system (primer, intermediate, topcoat) matched to the ISO 12944 corrosivity category and the service.
Buyers and maintenance managers often receive quotes that propose different systems for the same structure: a two-coat alkyd from one bidder, an epoxy-polyurethane from another. This guide explains what each resin does, where it fails and how to compare bids on an equal footing. It builds on our article on types of industrial coatings and our reference page on protective coatings.
What role does each coat play in a paint system?
Each coat has a specific job: the primer provides adhesion and, if it contains zinc, galvanic protection; the intermediate adds thickness and barrier effect; the topcoat resists the outside environment, UV and light abrasion. One resin can fill several of these roles, but rarely all of them equally well.
- Primer. Epoxy, zinc-rich epoxy, zinc silicate or alkyd. It must wet the prepared surface and tolerate the cleanliness grade actually achieved. See the zinc-rich primer entry.
- Intermediate. Almost always a high-solids epoxy, sometimes loaded with lamellar pigments that lengthen the path moisture must travel to reach the steel.
- Topcoat. Aliphatic polyurethane in most demanding exterior systems; alkyd or acrylic in mild environments; epoxy alone where the structure is shielded from UV or immersed.
Total thickness is measured coat by coat with non-destructive gauges and compared with the range set for each coat.
How do epoxy, polyurethane and alkyd differ?
They differ in how they cure, how they resist chemicals and how they hold up under UV. The table below summarizes generally recognized trends; each product's data sheet and the project specification take precedence.
| Criterion | Epoxy | Aliphatic polyurethane | Alkyd |
|---|---|---|---|
| Cure | Two-component reaction | Two-component reaction | Air oxidation, single component |
| Usual role | Primer, intermediate, immersion lining | Topcoat | Primer and topcoat in mild service |
| UV resistance | Poor: chalking and colour loss | Very good: colour and gloss retained | Moderate, yellows and dulls over time |
| Chemical and water resistance | Very good | Good for splash, not designed for immersion | Limited, sensitive to alkalis |
| Surface tolerance | Good for mastic-type epoxies | Needs a well-prepared substrate | Good on power-tool-cleaned steel |
| Cold-weather application | Limited depending on curing agent | Limited, sensitive to moisture | Slow drying in the cold |
| Relative first cost | Medium | Higher | Lowest |
The key point: epoxy and polyurethane work together, while alkyd is a separate, simpler system that lasts less in aggressive environments.
When two quotes propose different resins, check what each bidder assumed about surface preparation and exposure. An alkyd quote often assumes power tool cleaning and a mild environment; an epoxy-polyurethane quote usually assumes blasting and a harsher category. Comparing them on price alone compares two different scopes of work.
How do they perform in sunlight and UV?
Only aliphatic polyurethane (or an equivalent topcoat) keeps its appearance in full sun; epoxy chalks and fades, and alkyd dulls. Epoxy chalking is not first a protection failure: the film stays in place, but it wears at the surface and loses thickness over the long term.
- Visible exterior structure. Building steel, conveyors, tanks, walkways: a polyurethane topcoat protects the epoxy and keeps safety colours readable.
- Interior or buried structure. Epoxy alone is often enough, since UV does not reach it.
- Aromatic polyurethane. It also exists, but it yellows in sunlight and is kept for unexposed coats.
Which one resists chemicals and immersion?
Epoxy is the reference resin for immersion and moderate chemical exposure; polyurethane handles splash well but is not designed for immersion; alkyd suits neither. For acids or aggressive solvents, the choice moves to novolac epoxies or vinyl esters.
Alkyd has one precise limit: in alkaline conditions it saponifies. It must not go directly on fresh concrete, on a zinc-rich primer or on galvanized steel, where zinc corrosion products lift it. Our article on the duplex system on galvanized steel covers compatible primers.
Further reading: vinyl ester vs novolac epoxy in chemical service.
What typical systems apply by ISO 12944 category?
The corrosivity category steers the system: alkyd or acrylic in mild environments, epoxy-polyurethane in industrial environments, zinc-epoxy-polyurethane in severe environments, epoxy without a polyurethane topcoat in immersion. ISO 12944-5 gives reference systems by category and durability class; the project specification sets the thicknesses.
| ISO 12944 category | Typical environment | System often selected |
|---|---|---|
| C2 | Heated interior, rural atmosphere | Alkyd or acrylic, primer and topcoat |
| C3 | Urban atmosphere, low-humidity plant | Epoxy and polyurethane, or alkyd for low durability |
| C4 | Chemical plant, salted roadside | Epoxy or zinc-rich primer, epoxy intermediate, polyurethane topcoat |
| C5 | Humid heavy industry, marine | Zinc-rich primer, thick epoxy intermediate, polyurethane topcoat |
| CX | Extreme exposure, salt splash | High-build system defined case by case |
| Im1 to Im4 | Immersion, fresh or salt water, soil | Multi-coat epoxy or novolac epoxy, no UV topcoat |
Categories and durability classes are explained in our ISO 12944 explained guide, and typical builds in ISO 12944 paint systems C3, C4 and C5.
How do you compare the life-cycle cost of these options?
On a life-cycle basis, alkyd is the cheapest to buy but needs more frequent repainting in aggressive environments, while epoxy-polyurethane costs more up front and spaces out interventions. The right choice depends on the target service life, access to the structure and the cost of a shutdown.
The factors that weigh most, well ahead of the price per litre:
- Surface preparation. Abrasive blasting and access (scaffolding, lifts, containment) often make up most of a project's cost.
- Number of coats and recoat windows. A system with fewer coats can shorten the outage.
- Repainting frequency. Each intervention repeats access and mobilization costs.
- Cost of a production shutdown. It often exceeds the cost of the coating itself.
Our article on coating total cost of ownership gives a method to compare two quotes on that basis.
Can these resins be mixed during repainting?
Yes, as long as compatibility is respected: a polyurethane or mastic-type epoxy generally goes over a sound old alkyd, but a strong-solvent epoxy can lift an aged alkyd, and an alkyd must not go over a zinc primer. A compatibility trial on a test patch settles the question before the job.
Our article on painting over old coatings describes the test-patch method.
Frequently asked questions
Is alkyd still used in industry?
Yes, for dry interiors, lightly exposed structures and light maintenance on power-tool-cleaned steel. It is not recommended for immersion, over zinc, over galvanizing or in chemical service.
Can epoxy be left without a topcoat outdoors?
It can, if appearance matters little: protection stays in place, but the surface chalks and thins. A polyurethane topcoat is standard when the structure is visible or exposed to sun for long periods.
Can polyurethane be used as a primer?
Rarely. Most polyurethanes are topcoats; some moisture-cure polyurethanes serve as primers, but their use is decided with the data sheet and the specification.
Which system should a tender call for?
State the corrosivity category, the target durability class, the preparation grade and the thickness per coat. Quotes then become comparable, whatever manufacturer is proposed.
How do you get a quote for your structure?
The choice starts from the service environment, the target life and the preparation achievable on site. Our industrial painting service covers structures, tanks and equipment in Quebec and Ontario; our crews and qualified subcontractors work from LaSalle, including in Montreal and Ontario. Send photos, dimensions, service environment and the condition of the existing coating through the request a quote form: pricing on consultation, reply within 24 business hours.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.