What are these rust spots on Colorbond roof?

[Steve5]

I found about 15 places with small specs of rust towards the ends of 7m roofing panels.

Roof is fairly new so didn't expect something like this.

What causes it?  

How do you stop the rust without sanding down a section to bare metal for painting?

I want to avoid making a large area of new paint that isn't as strong as colorbond factory finish

Any thoughts greatly appreciated

[DIYGnome]

Hello @Steve5 these products (or any of your choice) claim to be designed/intended for use on painted metal (like Colorbond)


A rust converter product
https://www.bunnings.com.au/white-knight-0-125l-rust-guard-rust-converter_p0163911

A suitable priming product
https://www.bunnings.com.au/white-knight-rust-guard-400g-grey-sls-etch-primer-spray-paint_p1565449

A finishing top coat
https://www.bunnings.com.au/white-knight-rust-guard-300g-shale-grey-quick-dry-touch-up-spray-paint_p...


Blue Scope Steel in their Technical Bulletin TB-5 mention swarf (metal filaments from cutting/drilling) will cause staining/rust spots if not removed from the treated/painted metal surface.

Further reference
https://www.nsbluescope.com/my/wp-content/uploads/sites/7/2020/01/TB-5_R5_Nov03.pdf

[Steve5]

Thanks for the info on swarf.  i think that's the issue since it's only near the drill holes.

[MitchellMc]

Hi @Steve5,

I 100% agree with @DIYGnome that this is swarth left on the roof. The rusting is on the paint layer; it hasn't penetrated from beneath.  

I would suggest you clean the rust off the paint layer through the least aggressive means possible, and for now, don't apply any convertors, primers or paints to the roof. If you can't remove the rust with a mild solution of dishwashing soap and water and a soft sponge, then I would contact the installer and request they rectify the situation.

I don't believe you'll see any long-term effect from this rust that would compromise your roof sheeting. The metal shavings will rust completely away, and you'll be left with a rust stain, but that rust won't penetrate the paint and migrate to the internal metal of the roofing. In the worst-case scenario, you'll be left with staining on the roof.

Please let me know if you have any questions.

Mitchell

[Steve5]

Thanks!