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Life Span of Roof Paint

How to extend the life of a paint job

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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


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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