What is the best metal roof coating for an older metal roof?
Short answer. For an older exposed fastener metal roof with surface rust but sound panels, the usual choices are acrylic elastomeric coatings for sunny, well drained roofs, silicone for low spots where water sits, and urethanes where toughness matters, all over a rust inhibiting primer. Coating is the wrong fix if panels are rusted through, loose or bent.
Coating types
- Acrylic elastomeric: water based, reflective, UV stable and economical. Does not like standing water, so it suits sloped residential and barn roofs.
- Silicone: excellent UV and water resistance, handles ponding. Picks up dirt, and once applied, usually only silicone sticks to it later.
- Urethane: tough and impact resistant, often used as a base coat or on roofs with foot traffic. Costs more.
- Rust inhibiting primer: essential under any of them on bare or rusting steel.
Prep decides whether it lasts
- Clean off dirt, chalking and loose rust.
- Treat or prime rusted areas.
- Replace worn fasteners with new ones that have fresh sealing washers, often a size larger.
- Reinforce seams and fastener rows with fabric and base coat, if the system calls for it.
- Apply at the thickness the manufacturer specifies.
Georgia weather windows
Coatings need warm temperatures and dry conditions to cure. Heavy dew on humid nights and pop up afternoon storms can wash off or blister fresh coating, so follow the product's temperature and humidity limits.
When not to coat
- Panels with holes rusted through.
- Rotted purlins or decking underneath.
- A newer roof with an intact factory finish, which does not need coating.
See metal roofing in Georgia and common causes of roof leaks.