Rust spots on roof

[echnidna]

Found a couple of surface rust spots on my CGI roof.
Whats the best way to stop the rust speading?

[Bleedin Thumb]

Bob,
Sorry, I'm probably about to show my ignorance but what is a CGI?

[echnidna]

Corrugate Galvanised Iron (as distinct from Zincalume)

[Bleedin Thumb]

Thanks. I'm doing up my 4x4 car for sale at the moment. My favourite destination is Moreton Is so I'm dealing with rust. Been using a 90mm grinder to take off the flake and thick rust then spraying with Ranex RUSTBUSTER which is 35% phosphoric acid, leave it for 20 minutes and wipe off with a damp rag. It leaves the bare metal clean and sealed. You then have to prime and paint to match existing.
PS Ranex is available from Bunnies

[echnidna]

I'm not sure about using phosphoric acid on Galv sheet

[Bleedin Thumb]

I'm not sure about using phosphoric acid on Galv sheet

I read the bottle and it doesn't say not too, but I know what you mean. I recon you would be OK as long as you werent too liberal with it and really cleaned it off after its done its job. You can always do a test patch somewhere to see if it stuffs up the Gal.

[durwood]

You would have to leave the phosphoric acid on for a long time to get it to eat away the galvanising. It will mark it and clean it well but once the phosphoric acid hits the rusty steel it will kill the rust immediately.

Clean of loose rust with a wire brush/sand paper or what ever don't forget to remove any flacky gal also (rust underneath)

Apply the phosphoric acid mixed as per manufacturers instructions (most need watering down from a concentrate) as its only small bits put it on with a small brush or a rag. Most times you will be instructed to wipe over area with a damp cloth or hose off a large area, though some newer formulars you can let it dry. Then dry off the area, compressed air works best if not dry rag and heat ( out in the sun) In the case of your roof just wipe on wipe off let dry. (on a fine day.

Don't put your hands on the areas (perspiration) seal the bare metal with a suitable primer within 30 minutes. As its your roof cold gal would be best if its not painted, if it is, a good enamel primer ( cold gal preferably) and then roof colour.

Don't be temped to use concentrated Phos acid it will leave a sticky coat behind which will stop the paint sticking.