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Ask the Expert Question-and-Answer Archive by Ron Joseph June, 2006 Repairing Polyurethane Topcoat Finish Q. I have a tech question for you. I have an Upper Level Supervisor that is telling me that I am required to scuff sand and prime any polyurethane top coat that has been on for 48 hours or more cure time. The process that we have been doing is to sand the polyurethane painted surface to remove gloss wipe clean and re-paint. If the top coat is damaged or showing bare metal after surface sanding repairs and a conversion coating are done a wet coat of epoxy primer is sprayed to the bare metal and mist coat of primer is sprayed to the complete panel or seam to seam. After the primer has cured we re-paint the complete panel. Aged paint is always sanded and primed before top coating. I work in a paint shop that does a lot of Aircraft parts and there are times when a part needs to be re-painted like tiger stripes, dry, runs sags or primer showing through that in my opinion it does not need to be re-prime because it has been over 48 hours since top coated. Air Force T.O's are vague and address mostly the priming before top coat. I have Technical Data from PRC-DeSoto that backs up my process, but I would really value your input. A. Are you saying that when you redo parts that have tiger stripes, runs, sags,
dry spray, etc., you only get to those parts after 48 hours? If that is the
case, then the purpose of the epoxy primer is to provide adhesion between the
older polyurethane and the one you will now apply. The problem with poly is
that once it is fully cured adhesion becomes a problem. It is possible that
even without the primer you will do a good job, but since the aircraft flies
at incredibly fast speeds and the aircraft skin undergoes dramatic temperature
fluctuations it is probably safer to apply the epoxy before re-applying the
topcoat. Ron Joseph
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