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Dip painting galvanized tubes

I am dip painting 3" OD 18 gauge round tubes 7' long with a flat black paint. The tubes are galvanized. The paint finish looks great, however, it can be scraped off with your fingernail. How can I safely etch the galvanize coating before dipping to ensure paint adhesion? I was previously told to try cleaning the tubes with vinegar ⇦in bulk on eBay or Amazon [affil links] . It did help, but very little.

Thank you,

Thomas H. Mentecki
- Springdale, Pennsylvania
2002

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Hi, Thomas. The American Galvanizers Association at www.galvanizeit.com has a booklet on "Duplex Finishing" by which they mean galvanizing followed by painting. But in brief, galvanized surfaces which are intended to be the final finish have a chromate treatment which paint does not adhere to. When painting is intended, the chromate treatment is omitted.

I would clean the pipes in a hot slightly alkaline cleaner rather than with vinegar. I've also been told that scrubbing with a solution of trisodium phosphate ⇦ this on eBay or Amazon [affil links] will remove the chromate. Then apply a self-etching primer before the dip paint. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002