Flap Wheel grit size

Having never used these before and having to de-rust a large steel plate with modest pits, what is the most suitable grit size I should use?
This is for a 4.5" / 115mm angle grinder.
I also got the impression the finer grit sized wheels last longer?
Can anyone assist with pros and cons?
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Mike Perkins
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On Tuesday, 20 December 2016 22:58:30 UTC, Mike Perkins wrote:

Coarser is faster and gets more work done per wheel, but 40 grit would give you a scratched finish. It's mainly a question of what finish you want.
NT
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On 21/12/2016 00:15, snipped-for-privacy@gmail.com wrote:

The finish is not an issue, more the speed of rust removal!
Many thanks.
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On Tuesday, 20 December 2016 22:58:30 UTC, Mike Perkins wrote:

Wearing out is not an issue with flap wheels, they seem to go on forever. They work well, fast and leave the metal with a "bright" finish but also leave the surface with a rough "adzed" finish". You can't get into corners and crevices with them.
A wire "cup brush" is bit better in corners but doesn't leave a bright finish. It gets the surface rust off but the metal is left with a sort of brown film. Needs "Kurerust" or similar treatment prior to painting.
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Yes, I think I'd go with a wire brush for rust removal in general.
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Chris Green
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On 21/12/2016 10:55, Chris Green wrote:

Most two-pack epoxy paint suppliers suggest that wire-brushing is more likely to polish rust than remove it.
In the past I have used wire brush attachments and I am inclined to agree with this observation.
My instinct is that a flap wheel will be more aggressive with rust?
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I'm surprised, but open to persuasion. Most of my experience is on fairly badly pitted rusty metal, on this my feeling is that a wire brush will tend to get further into the pits. In general if this is the first stage of 'rust treatment' then whether the surface *looks* good isn't very important, what matters is if the 'paint' applied after can penetrate to solid metal.
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I had a rusty sunroof on the old Rover. One of the few panels you can't buy new.
Sanded it down to bare metal, then used a Dremel type thing with a diamond burr to grind out the pits. Then lead filled it. It cost 100 quid to have it painted, so wanted the paint to stay on as long as possible. ;-)
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Wednesday, 21 December 2016 20:06:24 UTC, Mike Perkins wrote:

IMLE a wire wheel removes most rust, but not all, it leaves a layer. OTOH it gets into pits better than sandpaper. So both might be best.
NT
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On 21/12/2016 23:18, snipped-for-privacy@gmail.com wrote:

That sounds a good plan.
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On 12/21/2016 10:55 AM, Chris Green wrote:

I usually find a "disk" type brush with twisted brushes a bit better in an Angle Grinder than a "Cup" brush. Agreed, you get a burnished finish with oxide embedded in the surface.
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On 21/12/2016 09:14, harry wrote:

It gets the surface rust off but the metal is left with a sort of brown film. Needs "Kurerust" or similar treatment prior to painting.

Kurust is now a useless product since it was been bought by Akzo Nobel (new owners of Dulux), and reformulated as water-based.
get it professionally shot blasted if at all possible and then paint or spray with zinc-rich primer immediately.
Akzo Nobel also now own hammerite and have turned that into just another paint. Previous formulation needed Xylene to clean the brushes. New version doesn't, and is nowhere near as good as it was. In fact you'll struggle to find any decent metal paints these days.
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On Thursday, 22 December 2016 23:54:45 UTC, Andrew wrote:

True about the Hammerite. Definitely smells different to the old stuff. Apparently Xylene is carcinogenic.
https://www.ncbi.nlm.nih.gov/pubmed/3053447
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It never was any use. Just one of those brands with a good name that gets remembered.
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worked fine on my Cortina. But that was 45 - 32 years ago
I suspect new formula is nothing to do with Akzo Nobel and a lot to do with EU requirements.
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And how long did it last before rust appeared again? The snag with any of these sort of things is knowing for sure it is any better than a plain decent primer.

Or makers hiding behind such 'requirements' as an excuse to make a cheaper product and sell it at a higher price.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Tuesday, 20 December 2016 22:58:30 UTC, Mike Perkins wrote:

Be sure to wear goggles BTW. Really essential.
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Mike Perkins was thinking very hard :

Electrolysis rust removal works rather well and only removes the rust.
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jim <k> wrote:

I've never found any chemical treatment of rust works as well as grinding back to bare clean steel. At best, it is merely better than painting over the rust.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 23/12/2016 11:18, Dave Plowman (News) wrote:

That also seems to be the consensus among paint manufacturers.
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