Painting a Metal Roof – DJ’s Painting

Coating a metal roof is not much different than painting a metal pole building. 90% of the time this metal is either a powder-coated, baked, paint finish or has a galvanized coating finish. These are great for long term outdoor exposure. Both systems hold up well to the UV and moisture over time. They also do a great job of protecting the metal from oxidation. These are the reasons why most people choose a metal roof.   

Metal manufactures claim that the metal never needs to be painted, but this is simply not the case. The sun's UV rays will break down the coatings over time, which exposes the metal. Also, the points where dissimilar metals touch each other (every fasten point where a screw or bolt touches the sheet metal) is a weak spot where rust will form.

When the coatings break down, the metal is left vulnerable, even if it’s just little areas here and there…  These areas start to oxidize or “rust” and break down. This can be very expensive to fix, not to mention it is just plain ugly… On walls, it will last anywhere from 15-20 years before the coating starts to fail. Roofs fail a little faster due to the more direct UV and rain exposures.  

Commercial steel roof painting.

Painting exterior metal facade on commercial grocery store in Rio Grande, NJ.

So now that we know that factory finished metal has a lifespan of 10-20 years, we can talk about how to maintain and extend its life.

First, be sure that the surface if clean and chalk free. To do this, it will need to be pressure washed. Then, the next step is to apply a complete prime coat. The material that best primes will vary depending on the situation we are dealing with. For baked finishes, which is a very hard coating and difficult for water based coatings to get a good bond, we use either a phenolic alkyd oil based bonding primer or a 2 part solvent based mastic epoxy bonding primer.

Although epoxy is the better choice, especially for rusted metal, the phenolic is less expensive and easier to use. A galvanized water-based bonding primer or epoxy is best since Alkyd can react with the zinc and cause the primer to lift after a few years…    

Based on the look, durability, and lifespan desired, there are some different choices concerning a top coat. For best color and gloss retention a 2 part aliphatic urethane top coat is best. This has a lifespan of 10 plus years. For a more economical coating, a DTM or waterborne single component urethane will work well and give about 5 years of good color retention. To keep costs down, we strongly recommend a good wash and repainting with the same top coat every 5-10 years.

It is best to re-coat every 5 years for single component top coats and every 10 years for two component top coats. Our reasoning for this is that on a roof or horizontal surface, the single component paints will “weather away” at a rate of about ½ a millimeter per year after the first year. But for a 2 part urethane, this process is a little slower. If the coating is allowed to weather away completely leaving the primer exposed, the primer will break down in just a few months under the suns UV rays. The rust will come back, requiring another prime coat and additional prep… Also, if the top coat is left to long, it may begin to lose adhesion. This can cause a problem for the future top coats for years to come…

Here's the gist, if you stay on top of the maintenance painting of your metal roof, it will save you time and money!