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Ask the Expert Question-and-Answer Archive

by Ron Joseph

February, 2010


Q. We are applying ARC S4+ with a Wiwa Plural component fleximix pump. We are doing a storage tank and on certain areas our paint seems to form these globhules which are up to 5mm in diammeter. It has cured 100% and there is no air or solvent intrapped. We are using a solvent free paint. How can these globhules form and what could be the cause? The mix ratio is also 100% and we couldn't identify any problems with the mix of the paint.

A. Off-hand I don't have an answer for you, but if you would send me a sample of the cured paint (a few square centimetres) I will look under the microscope and see if I can determine what these globules are.

There is an instrument that can scan a coating and look to see if the composition of the globules and the bulk of the coating are the same or different. The technique is called FTIR (Fourrier Transform Infrared Spectroscopy) and my laboratory has the ability to perform the test, if needed. It is not a forgone conclusion that FTIR will succeed. At the present time I too am examining a two-component, 92% solids epoxy coating for irregularities and I am struggling to get to the bottom of the problem. These are not always easy problems to resolve.

One of the tests I am performing is to draw down the coating with a doctor blade to see if the defect in my coating is reproducible in the absence of spraying. In other words, I'm trying to see if I can eliminate the spray gun or the spray application as the cause of the problem. So far no luck, but I think I'm making progress.

I don't know if there is a good paint laboratory in South Africa, but if there is you can send samples to the lab for further analysis. Otherwise, at the least, you can send me a sample for microscopic evaluation.


Ron Joseph

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