Pipe & Valve Coating

What sets pipe and valve coating apart

Hull and tank coatings are applied to large flat or gently curved steel areas and judged mainly on dry film thickness and adhesion. Piping and valve coating has to deal with threads, flanges, seat faces, small internal bores and, in sea chests and overboard lines, near-permanent immersion combined with biofouling pressure. The coating has to survive being masked around moving or mating parts, and in way of valves it must not be applied anywhere it could interfere with sealing or operation, which makes masking discipline as important as the paint itself.

Main components of the system

Surface preparation

Abrasive blasting or power tool cleaning to the specified standard before coating; residual mill scale or old coating left under a new system is the single most common cause of early failure.

Primer

Zinc-rich epoxy or equivalent primer for corrosion protection, chosen for compatibility with the intended topcoat and with the pipe's service, whether that is seawater, fuel, ballast or a dry utility line.

Intermediate and topcoat

Epoxy or polyurethane systems for external piping and exposed valve bodies; specialist foul-release or biocidal coatings for sea chests and overboard piping where marine growth restricts flow.

Masking of functional surfaces

Valve seats, stem threads, gasket faces and any machined mating surface are masked before coating and the masking is only removed after cure, since coating on a seat face causes a leak path that no amount of torque on the bolts will close.

Selection and sizing

• Service medium and temperature, since fuel and steam lines need a different resin system than seawater or bilge piping
• Immersion category, because permanently wetted sea chest and overboard piping needs antifouling performance that a dry cargo line does not
• Dry film thickness specified per coat and in total, checked with a calibrated gauge rather than assumed from the number of coats applied
• Compatibility with any cathodic protection already fitted in sea chests, since some coatings interact with sacrificial anodes

Regulations and class

There is no single dedicated SOLAS or MARPOL chapter for pipe and valve coating, but class rules require coatings in way of ballast and void spaces to meet recognised performance standards, and the IMO Performance Standard for Protective Coatings applies where ballast tank piping and supports are concerned. Antifouling coatings used on sea chests and overboard piping must comply with the AFS Convention's restrictions on biocidal active substances, which ruled out tributyltin systems and continues to restrict others.

Typical faults

What to look for in a supplier

• A written coating specification stating surface preparation standard, number of coats and dry film thickness per coat, not just a product name
• Confirmation that seat faces, threads and gaskets are masked as standard practice, with masking removal as a signed-off step
• Performance history or independent test data for antifouling coatings on sea chests, not only hull antifouling data applied by extension
• Batch traceability so a coating failure can be traced back to a specific application date and applicator

If a valve is hard to operate right after a coating job, do not force it — check for paint on the seat or stem before assuming it just needs breaking in.