Metal finishing Q&As since 1989

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Architect seeks best finish for exterior hand rails

I need to specifiy finishing for custom fabricated exterior handrails and guard rails in a subtropical environment with exposure to significant salt spray. I suggested epoxy paint over galvanized steel but was told that the epoxy would chalk. What would the best solution be? Powder Coating? Aliphatic polyurethane? Something else?

Jim Leslie
Landscape Architect - Westbrook, Maine, USA
2004

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2004

Dear Jim,

The quality and long term durability of the items for which you are to specify a suitable coating will depend upon following the following format:

The substrate: whether it be aluminium, mild steel, galvanized or stainless steel which all require pre-treating. This can be phosphating (zinc phosphate) for ferrous or chromating for aluminium.

Reason: This will slow down rust creep should at any time the surface coating is damaged.

Alternatively, here in the UK, coaters are using a waterborne primer which primer coats all substrates.

Surface coating depends on whether your customer requires a solid colour or a metallic but only powder coating will meet all the requirements of your client.

Solid colour: specify only polyester type or, even better, acrylic but acrylic powder is not used by many coaters as it can create problems with other types of powder they may be applying (will create pinholes in other types of applied powder -- but very good if you can find a coater who applies acrylic powders only).

Metallic colours: depending upon how the metallic is achieved (aluminium or mica raw material) it is imperative that if the colour is based upon aluminium flake you MUST insist upon a final coating of a polyester transparent (clear coat) which has UV absorbers incorporated into the formula being used (or the acrylic).

Of course a final coat of transparent over any colour would ensure that the life expectancy of the coating is improved (wear) but this would depend upon the cost your client is prepared to pay.

Terry Hickling
Birmingham, United Kingdom

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A epoxy primer with a urethane finish should do the job. Get primer with chrome in it. It will have a yellowish green colour. Aromatic urethanes are OK as long as they contain UV inhibitors. Powder coating is just an application method.

Ronald Zeeman
Coil Coating - Brampton, ON, Canada
2004