Concept · Coatings and corrosion

Protective coating types

How the common protective coating families cure and why, with the strengths, weaknesses and typical use of each, from zinc primers to polyurea.

Family	Cures by	Typical use	Watch for
Inorganic zinc silicate	Reaction with air moisture	Primer with galvanic protection, heat resistance	Dry air slows cure. Mud-cracks at high DFT.
Epoxy	Two-pack chemical reaction	Barrier coats, linings, intermediates	Chalks in sunlight. Cold slows cure. Amine blush.
Polyurethane (aliphatic)	Two-pack reaction	UV-stable, colour- and gloss-retaining topcoats	Isocyanate reacts with water, so humidity can cause bubbles.
Polysiloxane	Reaction, often with moisture	Durable, UV-stable topcoats	One-pack types cure with air moisture.
Polyurea	Very fast two-pack reaction	Thick, fast-setting membranes, plural-component spray	Very short working time.
Alkyd	Oxidation with air	Mild environments, maintenance painting	Thick coats wrinkle. Alkaline zinc attacks the binder.
Thermal spray aluminium or zinc	Not a cure: sprayed molten metal	Long-life barrier with sacrificial action, usually sealed	Porous as sprayed, so it needs a sealer.

The data sheet sets the surface preparation, film thickness, recoat window and cure conditions for each product.

The why: four ways a film forms

Solvent evaporation. Thermoplastic coatings such as vinyl and solvent-borne acrylic form a film as the solvent leaves. Nothing reacts, so the same solvent can dissolve the film again. A strong thinner in the next coat can make it wrinkle and lift. Water-borne paints dry by coalescence, and high humidity slows them.

Oxidation. An alkyd dries first by solvent loss. Then the drying oils in the binder take oxygen from the air and cross-link. The skin cures first and shrinks over soft paint below, so one very thick coat wrinkles.

Two-pack cross-linking. Base and hardener react after mixing. An epoxy base carries epoxy groups, and the hardener is a polyamine or polyamide. A polyurethane base carries hydroxyl groups, and the hardener is an isocyanate. The film cannot be dissolved again. The pot life is limited, so mix only what you can spray. The cure time depends on temperature, so cold slows it. See pot life.

Moisture cure. Water from the air is one of the reactants. Zinc silicate, one-pack polyurethane and one-pack polysiloxane cure this way. Isocyanate also reacts with water and releases carbon dioxide, so moisture in the air or on the surface can leave bubbles or pinholes in the film.

The coatings in the last three groups are convertible. They change chemically as they cure. A thermoplastic is non-convertible. The moisture rule is not the same for every family. High humidity can hurt an epoxy or a polyurethane, but zinc silicate needs humidity to cure. Always read the data sheet for the product in use.

Strengths and weaknesses

Epoxy: the usual barrier coat, from primers and intermediates to linings. Ballast tank coatings under IMO PSPC are epoxy-based. Most epoxies chalk in sunlight, so a topcoat keeps colour and gloss. Cold and humid curing raise the risk of amine blush, which weakens the next coat.
Polyurethane: aliphatic types keep gloss and colour. Aromatic types tend to chalk, so topcoats are aliphatic. Polysiloxanes, polyaspartates and FEVE fluoropolymers can replace polyurethane topcoats.
Polysiloxane: it has an inorganic silicone part and an organic part. The silicon–oxygen backbone resists UV, so the gloss and colour last.
Zinc silicate: it protects galvanically and resists heat to about 400 °C. It needs humidity and a limited film thickness. See zinc-rich primers.
Alkyd: it suits mild environments and maintenance painting. Zinc and its corrosion products are alkaline and attack the oil binder, so an alkyd on new galvanizing often peels.
Thermal spray: the metal bonds mechanically. It needs a very clean, coarse profile. See ISO 2063.

In the field

Check the product name, batch and generic type against the specification and the data sheet.
Read the cure conditions: minimum and maximum temperature, humidity limit, pot life and recoat window.
Measure the climate at the start and during the work. See dew point.
Check the mixing ratio and the pot life for each batch.
Before a recoat, look at the old coat for blush, chalk and contamination. See cure and recoat.
Where the specification asks, verify the cure before topcoating. For zinc silicate, use the MEK rub test of ASTM D4752.

What the inspector records

Record the product, batch, mixing ratio and mixing time. Record the climate, the pot life used, the interval between coats and any cure test result.

Exam traps

Trap: A film that is dry to the touch is not necessarily cured.
Trap: An epoxy does not keep its colour in sunlight. It chalks. Use a polyurethane or polysiloxane topcoat where colour matters.
Trap: An alkyd on new galvanizing peels because of the alkaline surface.
Trap: A thermoplastic coating can dissolve again in its own solvent. Epoxies, polyurethanes and zinc silicates are convertible.
Trap: Thermal spray metal has no cure. It is sprayed molten metal.
Trap: High humidity is not bad for every coating. Zinc silicate needs it.

How the exams treat it

The FROSIO book covers paint composition, generic types, systems and application. Coatings and inspection is 20 % of the CIP 1 blueprint. CIP 2 groups coatings and coating application at 13 %.

142 practice items in the members area mention this topic (CIP 1: 38 · CIP 2: 56 · FROSIO written: 48).

Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.

Take the free readiness check See the tracks

About this page

Sources: ISO 12944-5:2019 · ASTM D4752-20(2024) · ISO 2063-1:2019 · ISO 2063-2:2017 · FROSIO Surface Treatment Book · KTA-Tator, Certified Coating Inspector Forum, June 2023.

Written from the published standards and coating practice, in our own words, and checked on 2026-10-04. Found an error? It goes in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.