Ask the Expert Question-and-Answer Archive

by Ron Joseph

July, 2001

Painting Over Existing Surfaces

Q. I would like information about equiptment and the process necessary to paint exterior corrugated metal buildings. The coating is primarily for protection against corrosion

A. Every situation is different and one recommendation is not valid for all. In "general" the surfaces will need to be thoroughly cleaned. If you have a power washer with detergent, this will help to get all the dirt and loose paint off the corrugated metal sidings. After using the detergent solution you will need to rinse with clean tap water.

You might need to scuff sand areas where the old paint has been oxidized. If you don't remove the heavy oxide buildup, the new paint might not adhere, and sooner or later is will flake off from the building. After scuff sanding you will need to wash the areas clean of all powdery deposits.

If you see rust coming through the paint, you will need to sand down those areas to remove the rust and expose clean bare metal. Insure that you feather the edges between the intact paint and the rusty areas. Then immediately apply a corrosion resistant primer that you can purchase from a paint supply store. Again, insure that you apply the primer so that it overlaps approximately 1" onto the intact paint.

Finally, when all the prep work has been completed apply one or preferably two coats of a high quality exterior latex paint over the entire surface.

Please be aware that each building is different and needs to be approached differently. The paint on the sidings that face the sun might have more of an oxide buildup than the paint that is shaded from the sun. Therefore, if you need more advice, it might be prudent to ask your local paint rep to examine the building before you start.