My understanding is that zinc rich primer should only ever be the first coat as that is the only place it is advantageous and that's only on SSPC-6 or 10 metal. You don't get the galvanic benefits without a tight contact and the zinc primers don't have as good of adhesion in subsequent coats.

Dear Tugboat,

Here Question is not about overcoating phenolic epoxy with zinc.
Between first and top shells of floating roof tank, why zinc rich primer is applied on blast cleaned shell substrate.

I hope misinformation corrected.

Zinc rich primers provide a slight galvanic protection to the substrate. This requires a tight electrical connection which requires a blasted or acid etched surface. Other coatings and oxides prevent the galvanic connection. On our marine applications, we may specify two coats of International 300V (not zinc rich) as anti-corrosion coatings below the waterline as sacrificial anodes provide the galvanic corrosion protection. Above the waterline we use one coat of Interzinc 75V (zinc rich) topped with at least one coat of 300V. None of these are final coats.

Epoxy paints are highly sensitive to a few conditions including oil contamination and extended recoat intervals so of flaking is your issue I would start there.

Who knows the vagaries of the specifier? It would seem that more aggressive corrosion is expected at the first and top courses, whereas, in the intermediate courses, someone might have had the idea of simply stopping any light corrosion contaminating the product. Sometimes, end users are helpful in that they go some way to explaining why they specify what they do in the specification itself.

Steve Jones
Corrosion Management Consultant

www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.