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Acid as Primer on Galvanized Surface?
for Engineers, Shops, Specifiers
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"User's Guide to Hot Dip Galvanizing for Corrosion Protection in Atmospheric Service" by NACE (1997 only rarely avail.)
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Q. 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 ⇦in bulk on eBay or Amazon [affil links] -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. AbramsEngineer - Queens, New York, USA
October 24, 2010
A. 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 it's 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
A. Geoff is polite enough to not tell the world "I told you so" :-)
When the galvanizing he does is intended to be painted his shop does zinc phosphatizing, which is the proper pretreatment of galvanizing for subsequent painting.
When zinc phosphatizing has not been done, either because someone changed their mind about painting it, or had wanted to leave the option open, or tried to save costs, you are left with trying to gain adhesion via acid etching and/or self-etching primers. When you're lucky, it works ... but it's not the robust solution that zinc phosphatizing is :-)
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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