Treating rust on galvanised steel roof

So I'm restoring our 40yo galvanised flat roof. 80% surface rust, no serious damage.
Mostly surface rust surrounded by patches of original gal on the unrusted areas. So a combination of both rusty steel and galvanised steel.

This is my proposed paint system
- Wash and pressure clean, allow dry
- Apply rust converter – wait 24 hours
- Pressure clean again to neutralise acid
- Apply zinc phosphate metal primer (1-2 coats)
- Apply Dulux Roof and Trim top coat (1-2 coats)

So I'm using Ranex Rustbuster to treat the rusty areas. As a test patch I pressure cleaned, then washed with soap, rinsed and dried to remove any salt or oil that might affect the chemical reaction. As soon as it had dried thoroughly I applied a liberal amount of rust converter on a 2m test patch and left it for 24 hours. I've had mixed results.

Some of the rust has gone white (what it's meant to do). On much of the rest the red rust colour is still visible. There's also a white powdery coating all over the surface.

So I called Bondall who recommend that Rustbuster not be used on Galvanised surfaces. The white powder is actually the acid etching into the surrounding gal. Should I be concerned by this? If anything won't it better prepare the surface for priming?

Also am I correct in thinking that the rusted areas no longer have any gal, that's why they are rusting? Therefore shouldn't the Rustbuster work on the rusty parts? They suggested applying another coat to the areas that were still rusty, but shouldn't this stuff work almost instantly on light surface rust? It's not exactly cheap and I don't have enough of it (or the time) to apply two (or three) coats to the entire roof. What might be stopping it from working effectively? Could it be the galvanised surface? Any ideas?

Would love to hear any tips from those who have restored rusty gal roofs.

cheers

Not sure if this may help you or not but instead of going through all that since you are painting the roof why not paint it with liquid rubber then paint over top with what ever color you want.
Google liquid rubber in qld and look at there videos just a thought.i have use it on a lot of roofs to repair from leaking when customers of mine dont have enough for a reroof

Thanks for the suggestion. But what about treating the rust? Surely that needs to be treated first to prevent the rust from getting worse underneath the rubber membrane.

Any patch job you do will only buy time.
40yo gal roof is towards the end of its life.

Reason being, even if you treat what you can see, there will be plenty that you can't see, and because of its position, it is generally more severe.
Prepare for replacement if possible.
Sorry to be the bearer of bad news.

We are planning a reno in the next 5 years that will more than likely involve removal/replacement of the existing skillion roof. So time is exactly what we need to buy right now.

I've successfully de-rusted some of my older no longer used drill bits and chisels by soaking them in a bath of lemon juice for 24hrs. It worked really well and simply converts the rust into a black goop that can be washed off. It took the rust back to bare steel. Maybe try some on a patch and see how it goes. It would certainly be just as effective and probably 90% cheaper than any rust converter I've seen. Once back to bare steel just a good metal primer and paint.

Stewie

If you are happy to buy time and prepared to replace in a few years, i'd suggest you grab some White Knight rust guard.
It now comes in a single step formula so no priming needed.
The only drama with it is that it is not UV resistant, so I would suggest checking condition again in 12 months and potentially repainting if neccesary.
This is what I use for people wanting to buy time while they can plan for replacement.
Lightly brush the roof down with a wire brush, wash with water, and give 2 coats when dry.

The issue with trying to treat all the rust and get any longevity out of it is that you will have rust on the under sides of the roof, gutter, nails etc as well. So to to be sure of killing ALL rust, you would need to remove roof, gutters, screws/nails etc.

So keep it simple, rust guard will buy some time.

All the best with it bud.