Q:

My house has a metal roof. The metal is good, but there’s a lot of surface rust. I’ve had it painted twice in the past 30 years, most recently about six years ago. I think the painters sprayed on an oil-based paint. It looked good for a while, but now there’s a lot of rust coming through. Do you have any suggestions?

Adam Huffer, Louisville, KY

A:

The paint could be failing for one of several reasons: sloppy prep work, the wrong product, or improper application. Unless there are other problems, however, it will be more economical to have the roof painted a third time than to replace it. That’s what I gathered from talking with several experts, including Todd Miller, president of Classic Metal Roofing Systems and a founder of the Metal Roofing Alliance, a trade group for the residential metal-roofing industry. According to Miller, a repainted roof should last as long as 20 years.

Thorough preparation is key. Any loose paint—and in this case, rust—should be cleaned from the surface before a new coating is applied. Miller suggests powerwashing the roof, taking care not to spray directly into any points where roof panels overlap. Patches of rust should be removed with a wire brush and coated with a rust-inhibiting primer.

When it comes to choosing the primer and topcoat, you could go to a good paint store and ask for products that are specifically formulated for metal roofs. The best finishes, Miller says, are fluoropolymers, used for new metal roofing under the trade names Kynar 500 and Hylar 5000. Chemically, they are polyvinylidene difluoride, or PVDF, and according to Miller, will last up to 50 years when factory-applied to new roofing. The trouble is that you can’t duplicate factory conditions for applying the paint. It’s baked on.

The resin that goes into Kynar 500 is made by Arkema, which sells it to paint manufacturers that actually make the paint. Arkema also produces Kynar Aquatec, a water-based formulation designed for field application on metal. Paint containing this resin is produced by a number of companies, including The Garland Co., United Coatings, and APV Engineered Coatings. An Aquatec coating should last for 20 years, but it’s not cheap. Expect to pay $100 a gallon for it.

One word of caution: Depending on what type of paint is already on the roof, it may be necessary to scuff-sand the surface to make sure the new topcoat bonds. APV’s Mike Couchie says Aquatec has trouble sticking to silicone polyester paints without this extra step.

Professionals often apply the coatings with sprayers and then roll them out. It’s not a job for most homeowners. The Metal Roofing Alliance maintains a database of roofing contractors, searchable by ZIP code, at www.metalroofing.com.