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Acid as Primer on Galvanized Surface?

October 24, 2010

Hi, we're a construction supervising company and the contractor installed galvanized guard rails for train trestle. Prior to painting, the contractor primed the surface by brushing on a vinegar-type acid in order for the paint to adhere better to the galvanized surface. I noticed black drippings where they brushed. Did they do more harm than good, or is the surface now appropriately primed for the paint?

B. Abrams
Engineer - Queens, New York, USA

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October 26, 2010

They probably brushed on a phosphoric acid mixture that has an additive that causes it to turn black. There was a mixture for this priming job developed decades back (goes under names like "T Wash") The blackness was for the supervisor to be sure it was applied.
If its that, they did a good thing.
Painting onto fresh galvanizing without some form of etch primer or similar is futile, it will not stick.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK