Q: About four years ago, I painted a galvanized metal roof using oil-based primer and an exterior oil-based top coat, as per paint supplier recommendations. After two years, most of the top coat came off, although the primer held fast. I repainted with the same top coat after a water blasting, TSP cleaning, and a second water blast. And after two years, it is failing again. What can I do to increase the life of my paint job? — Edgar, Cardigan Pei, Canada

A: Steve Thomas replies: First, for those who may not know, TSP is trisodium phosphate, a very strong detergent. If you have carefully followed the manufacturer's instructions for your oil-based paint but it is not continuing to adhere to the primer, although the primer does adhere to the substrate, then there would appear to be a poor chemical or poor mechanical bond between the primer and the top coat. If I were in your shoes, I would first call the technical assistance line at the company that manufactured the primer and ask them what top coat system they would recommend. They know the chemistry of their product and they should know what bonds with what, and where the problem might lie. This is a strategy that I've personally used with complex coating situations such as on sailboats and marine coatings, and it has always worked well for me.

I'd also note that we've had great luck here in New England using latex top coat over oil-based primer, so you may want to try that. Good luck.
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