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April 30 - May 6, 2012

How do you determine whether a lead-bearing coating can be overcoated or removed and replaced?

Selected Answers

From Frank Rea of GPI-Southeast on May 1, 2012:

     To determine whether any coating can be overcoated or needs to be removed and replaced, one must first conduct an existing coatings condition assessment. The assessment consists of observations and tests which provide the data necessary to make the decision. A typical condition assessment begins with observing the type, amount, and distribution of rust and coatings defects. SSPC-Vis 2 is a helpful guide when making this determination. Typically, if the amount of rust exceeds 25% or is destributed randomly over several locations, full removal is likely because spot cleaning would be too labor-intensive and time-consuming.   

   Second, dry film thickness and number of coats should be measured with a magnetic DFT gage and a Tooke gage at random and representative locatons. There are only so many layers and a maximum total thickness that can be applied before risking failure. Next, utilizing ASTM D 3359 Methods A (DFT > 5 mils) or B (DFT < 5 mils), determine the adhesion rating. When overcoating, the new paint may shrink as it cures and put stress on the existing coating. If the existing coating is not tightly adhered, it may peel. Lastly, take sample of the paint and have the top coat analyzed by Fourier Transform Infrared Spectroscopy (FTIR) to verify the generic type of resin. This will allow the specifier to select a compatible overcoat material.

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