Surface rust refers to the early stages of rust formation where it only affects the surface layer of a metal object. At this stage, the rust has not penetrated deeply into the metal. Painting over surface rust is often done, but there are some important factors to consider before doing so.
What causes surface rust to form?
Rust forms when iron or steel is exposed to oxygen and moisture. When iron oxidizes, it forms rust. The chemical reaction requires all three elements – iron, oxygen, and water. When all three are present, the iron begins corroding leading to the familiar reddish-brown rust.
On a newly formed surface rust spot, only the topmost layer of the metal is affected. With time and continued exposure to the elements, surface rust can spread both outwards across the surface as well as inwards into the metal. Preventing further spreading of the rust requires removing or effectively sealing the surface rust.
Is painting over surface rust an effective solution?
Painting over surface rust can sometimes temporarily halt the spread of corrosion. However, it is generally not a permanent solution and will ultimately fail over time. There are a few reasons for this:
- Paint does not stop the chemical reaction – Rust forms from an electrochemical process that requires water and oxygen. Paint simply forms a physical barrier and does not address the chemical aspects.
- Rust continues under the paint – Even if painted over, the rust continues to spread underneath the paint, eventually causing cracks, bubbles, and peeling.
- Surface preparation is key – For paint to properly adhere to a surface, proper surface prep is vital. But it’s difficult to properly prepare a rusty surface without removing all the rust first.
So while paint may seem like an easy short-term cover-up for surface rust, it is not going to stop the rusting process. Any trapped moisture under the paint will continue to spread the rust under the painted surface.
When is it OK to paint over surface rust?
Painting over surface rust is realistically only a short-term solution in a handful of scenarios:
- When preventing further rust spread is not critical – Such as a beater car that will be used for only another year or so anyway.
- As a temporary protective layer – When a more permanent rust treatment isn’t immediately feasible, paint may help buy some time.
- For aesthetic purposes only – Such as when appearance is important but corrosion prevention is not, like a decorative feature.
However, keep in mind that even in these cases, the rust will continue to spread under the paint. And once severe bubbling or cracking appears, the paint will have to be fully removed anyway.
What are the steps to properly paint over surface rust?
If you do need to paint over surface rust, follow these steps to maximize the temporary protective ability of the paint:
- Clean the surface – Remove all dirt, grease, salts, and loose rust particles. Wipe clean with a degreaser.
- Sand the area – Lightly sand to roughen up the surface for better paint adhesion.
- Apply rust converter – Use a rust converter chemical treatment to stabilize the rust.
- Use a rust-inhibitive primer – Priming with a rust-preventative primer will add another layer of protection.
- Apply topcoat – Use an exterior grade enamel paint for the topcoat.
- Monitor and reapply – Check regularly for cracking or bubbling and reapply as needed.
What are some alternatives to painting over surface rust?
Instead of just covering up surface rust with paint, consider these more permanent solutions:
Remove the rust
Removing all the surface rust through sanding, grinding, wire brushing, or sandblasting can fully reset the metal’s surface. Ensure it’s wiped with a rust converter chemical to neutralize any remaining traces.
Rust converter
Rust converter chemicals convert existing rust into an inert protective layer while also inhibiting further corrosion. Can be applied directly over surface rust.
Rust removers
Rust removal chemicals dissolve rust and convert it to a water-soluble layer that can be rinsed off, leaving clean metal below. Effective for small surface rust areas.
Rust-inhibiting primer
Priming with specialty rust-inhibiting primers after thorough surface prep creates a protective under-layer that prevents further rust formation.
Leave as-is
On non-critical decorative items, an alternative is to simply leave the existing surface rust intact and prevent further rusting. Clear sealants can stabilize rust and prevent flakes.
Key takeaways on painting over surface rust
- Painting over rust only hides the problem rather than solving it.
- Rust continues spreading under paint, eventually causing it to fail.
- Proper surface preparation is critical, but difficult over rust.
- Painting rust should only be a temporary fix or for appearance only.
- For a permanent fix, rust must be fully removed or treated with converters.
Conclusion
Painting over surface rust without properly removing or treating it first is not an effective long-term solution. The rust will continue spreading underneath the paint and eventually lead to failure. For critical metal surfaces, it is better to fully remove the rust or use specialty rust conversion/prevention products. However, on decorative or non-critical items, paint may be used as a temporary fix or to simply improve appearance. Just keep in mind that repainting will be required as the rust continues expanding under the surface.