How to Choose an Anti-Rust Coating System for Steel

Step 1: Know your environment

The same steel gate needs a very different system indoors, in a city, near the coast or inside a chemical plant. Coastal air, humidity and industrial fumes speed up corrosion, so describe where the steel is and what it is exposed to.

Also tell us whether the steel will be immersed in water or buried. Some finishes are not designed for continuous immersion. For example, the Colythane 300 sheet states it is not designed for continuous water immersion. Structures at or below the waterline need a system such as ColyPotar 520.

Step 2: Identify the metal and its condition

• New steel or rusty steel: rust and mill scale must be removed to the level the system requires.
• Galvanised or stainless steel: these smooth surfaces need an adhesion-promoting primer such as Etching Primer.
• Previously painted steel: check whether the old coating is sound and compatible. Surface-tolerant epoxies such as ColyMastic 850 are designed for maintenance work, but surface tolerance does not remove the need for preparation.

Step 3: Build the system

A typical protective system has three roles:

1. Primer: the first protective layer and the foundation for adhesion. Options: Oxide Primer for ferrous metal, Colypoxy 502 (satin) or Colypoxy 505 (glossy), both two-component epoxy universal primers listed for steel, galvanised surfaces, aluminium, stainless steel, FRP and wood.
2. Intermediate or barrier coat: adds film build to slow down moisture and oxygen. Options: PG Express 555 Red Oxide – PG 555 or PG Express 555 MIO – PG 123, high-build epoxies with surface tolerance for atmospheric exposure, or Micaceous Iron Oxide.
3. Finish: provides weathering resistance, colour and gloss. Option: Colythane 300, a high-gloss acrylic polyurethane listed for offshore structures, petrochemical plants, bridges and power plants.

Some products can play more than one role. The ColyMastic range and PG Express 555 can be used as a primer, intermediate or finishing coat, depending on the approved system. For better exterior weathering, the ColyMastic sheet recommends a compatible polyurethane finish.

Step 4: Plan for recoating

Ask about overcoating intervals and future maintenance when you choose the system. A finish with good recoatability makes future touch-ups simpler. Colythane 300 is designed for long-term recoatability.

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Common mistakes to avoid

• Painting over loose rust or mill scale.
• Using an ordinary primer on galvanised steel.
• Mixing products from different systems without checking compatibility.
• Applying too thin a film to save paint.
• Using an atmospheric finish on steel that will be immersed or buried.

Questions we often get