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Powder Coating Versus Galvanizing with Top Coating of Paint

We are working in a marine climate on a stair project that, while covered, will be subject to high use and moisture. The Architectural Painting Specification Manual indicates that in higher traffic conditions we can utilize galvanized metal with a Pigmented Polyurethane over epoxy primer. Is this the best solution for such a situation or would Powder Coating provide better endurance? I understand that there is a third, "combined" option of applying Powder Coating over a galvanized surface. Cost and maintenance are both issues for the project. Is there a significant cost difference? Is one type easier to repaint or repair if scratches or denting occurs?

Thanks for any help you can offer!

Katherine Aulwes
Architect - Portland, Oregon, USA
July 21, 2008

Hi, Katherine. Powder coating, as a general rule, is a bit more corrosion resistant than liquid paint, primarily because it tends to be thicker. But ecology and ease of capture and reuse are probably stronger driving factors in selecting powder over liquid paint than corrosion resistance is.

Galvanizing, even without any paint, can often last 50 years. So there is no comparison between powder coating and galvanizing plus paint of any sort. Galvanizing coats the steel with zinc, offering galvanic protection (protection even if the coating is scratched through. Powder coating doesn't offer this. I do not denigrate powder coating, it's a wonderful finish for many things; but for a stairway in a moist and salty environment it will not even compare with galvanizing plus paint. But remember, this is simply my opinion and experience. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 22, 2008

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First of two simultaneous responses -- July 23, 2008

Galvanizing will last to 50 years or so that Ted mentions above. Alone without any other coating it will do this, and its wear characteristics are better than most paints, so on the steps it'd be good.

But if you need an aesthetic finish too, then either paint or powdercoating over the galvanizing is possible, but be sure to choose a coater that has a competent pretreatment system. (Chromate, Zinc Phosphate etc, done in multiple stages, often the more the better). If the surface of the galvanizing is not pretreated competently the paint/powder will flake off in no time. If done right, it will extend the life of the galv, and you should get perhaps 25-30 years maintenance free on the paint/powder finish, and still have no corrosion.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

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Second of two simultaneous responses --

Hi, Katherine. Ted's reply is correct. However, the third option you may be referring too could be a finishing process called "Duplex" coating. This name covers a process that combines the weathering properties of a galvanised substrate and the aesthetic (various colours) value of polyester powders. Some advocates of "Duplex" coating state that it adds a further 15 years to the life span of the coating system. So if you are seeking to get away from that ultimate dull grey galvanised finish, consider over-coating it with a polyester powder.

Terry Hickling
Birmingham, United Kingdom
July 24, 2008

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Hi,

its quite surprising to know that galvanized coatings lasts up to 50 years in marine atmosphere.
I have seen coatings being corroded in a months time too well in dry atmosphere and country side both Mr.TED and Mr.Geoff are right but not definitely in an environment what Miss. Katherine is mentioning according to my experience.
for this kind, a duplex coating as suggested by both will be good, either in the form of powder coating or painting after hot dip galvanizing.

Best Regards,

S.R.Ameen
- U.A.E.
August 26, 2008