ASTM A780-Zinc Dust in Paint
ASTM A780 states zinc-rich paint with zinc dust in the ranges of 65-69% or above 92% in the dried film are equally effective. Why is there a gap in the amount of zinc dust in the paints listed in ASTM A780?
Paints containing zinc dust, with concentrations of zinc dust in the range of 65 to 69% or above 92% in the dried film, are considered equally effective for the repair of damaged galvanized coatings.
A common mistake when interpreting this information is the 65% to 69% zinc dust concentration and the 92% concentration are from the same sample of paint. This is not the case. Rather, the excerpt above gives two different concentrations of zinc dust in zinc-rich paint, which both performed equally well during testing. In other words, if you were to choose to make repairs on your galvanized steel with a zinc-rich paint, you could use a paint with a concentration of zinc dust in the range of 65 to 69% OR a zinc rich paint with a zinc dust concentration of 92% or greater.
So why do zinc-rich paints with zinc dust concentrations outside the recommended ranges not work as well?
In several studies that looked at the efficacy of zinc-rich paints with various zinc dust concentrations, the data showed paints with concentrations in the range of above 69% to below 92% didnt perform as well as paints with zinc dust concentrations in the ranges of 65 to 69% or above 92%. In some cases, this was because paints with zinc dust concentrations outside the recommended ranges didnt adhere to the steel. For more information, you can request copies of the studies that tested the performance of paints with various zinc dust concentrations by sending an email to firstname.lastname@example.org and requesting the zinc-rich paint loading studies.
© 2019 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.