Zinc-rich epoxy, zinc silicate or zinc-free epoxy? How to pick the primer layer for C5-M and offshore steel by service life, surface prep and recoat window.
The primer is the layer that actually stops corrosion. Topcoats decide how the structure looks and how well it resists UV and chemicals, but if the primer layer fails, the whole system fails with it. In C5-M and CX environments — offshore, coastal, heavy industrial — the primer choice is worth more attention than the topcoat brand.
Start from the exposure, not from the brand
Work through four questions before you look at any product list:
- What is the corrosivity category? C5-M (coastal and offshore atmosphere) already demands a zinc-based primer in most system tables; CX usually means a zinc silicate or a two-coat zinc-rich build with a high total DFT.
- What surface preparation can the yard actually achieve? Zinc silicate needs a sharp, well-graded profile and is unforgiving of contamination. If the blasting standard is uncertain, a zinc-rich epoxy is the safer route.
- How long is the steel exposed before topcoating? Long fabrication and storage periods punish primers with short maximum exposure times.
- What is the target service life? 15 years, 25 years and 25+ years map to visibly different total dry film thickness and, often, to a different primer family.
The three primer families and where they fit
- Zinc-rich epoxy — the workhorse for C4 to C5-M. Tolerant of slightly less perfect preparation, compatible with most intermediate coats, and the most common choice for fabricated steel that must sit in storage before topcoating.
- Inorganic / ethyl zinc silicate — highest level of cathodic protection per unit thickness and excellent heat resistance. Best on well-prepared steel in C5-M and CX, and the usual choice for tanks and hot surfaces.
- Zinc-free epoxy — used where zinc cannot be applied, for example on stainless steel, aluminium, or where welding and cutting fumes or a downstream process make zinc undesirable. It protects by barrier action only, so it needs a thicker total system.
What usually goes wrong
- Specifying a zinc silicate on steel that has been blasted to a lower standard or has salt contamination — the coating will not develop a proper bond and will fail early.
- Ignoring the maximum overcoating interval. Many zinc-rich primers develop a hard, glossy, zinc-carbonate surface if left too long; the intermediate coat then adheres poorly.
- Comparing systems by total thickness only. 240 µm of the wrong primer family performs worse than 200 µm of the right one.
- Skipping the stripe coat on welds, edges and cut-outs. In C5-M exposure, more than half of early corrosion starts at these details, not on the flat plate.
A practical sequence
Fix the corrosivity category and target life first, then the achievable surface preparation, then the exposure window during fabrication. Only after those three are settled does the primer family become obvious — and only then is it worth comparing products across brands on price.