Guide
Galvanising or powder coating: choosing a finish for fabricated steel
Updated
Finish is the decision most often left until the part is made, and it is the one that most often has to be made before the part is cut.
The options
- Hot dip galvanising
- The part is dipped in molten zinc, giving a metallurgically bonded coating inside and out. The best general protection for outdoor steel, priced by weight, with a grey finish that is functional rather than decorative.
- Powder coating
- Electrostatically applied powder, cured in an oven. Wide colour range, hard finish, priced by area. Protection depends on the surface preparation underneath it as much as the powder itself.
- Wet paint
- Priced by area and specification. Flexible for large or awkward items that will not fit in an oven or a galvanising bath, and the only practical option for touching up on site.
- Duplex (galvanise then powder coat)
- Zinc protection plus a colour finish, and the longest-lasting of the common systems. It is two processes, two lots of transport and two lead times, and needs the coater to prepare the galvanised surface properly.
The design details you must settle before fabrication
- Vent and drainage holes for galvanising. Hollow sections must be able to fill, vent and drain in the bath. If they are not designed in, they get drilled somewhere you did not choose, or the part cannot be dipped at all.
- Size against the bath or the oven. Both galvanising baths and coating ovens have physical limits. Oversized items are double dipped or painted instead, which changes both price and appearance.
- Distortion. Dipping thin or asymmetric fabrications into molten zinc puts heat into them. Thin sheet assemblies can move.
- Threads and machined faces. Coating thickness matters on anything that must fit. Say which faces must be masked or cleaned after coating.
- Colour reference. Give a RAL or BS reference, not a colour name. "Dark grey" is not a specification.
How finish shows up in the quote
Galvanising is charged by weight, so it scales with how heavy the part is rather than how big it looks. Powder coating is charged by area, so a large thin panel costs more to coat than a compact heavy bracket. Both are usually subcontracted, which means transport each way and a lead time that is outside the fabricator's control. If your programme is tight, ask what the finishing lead time is before you commit, because it is a common reason for late delivery.
Matching the finish to the environment
Indoors and dry, powder coating over properly prepared steel is usually plenty. Outdoors, galvanising or duplex is the durable answer. Coastal, industrial or wet environments, and anything in contact with the ground or with de-icing salt, need a deliberate specification rather than a default. If you do not know, describe the environment to the fabricator and ask what they would specify, then get that in writing on the quote so every price covers the same system.
Stainless steel is not automatically maintenance free, and grade matters: a grade suitable for an internal handrail may not be right for a coastal balustrade. Say where the part will live.