I am buying a house that requires some minor modifications. One of them is a recommendation to remove a small area of surface rust on a metal roof. I'm interested in the best way to achieve this.
There at multiple products with which to remove rust from iron based metals. There are the mechanical methods which include sanding or wire brushing. Both of these methods can be done by hand or with a power tool.
The other method is to use a rust remover chemical. The common product is often referred to as "naval jelly". It is brushed on and then let sit for a while and then washed away. Often it is necessary to use a combination of approaches, the jelly and some concurrent work with steel wool or a wire brush. Make sure to use proper personal protection when working with the chemicals.
Another very important part of removing rust is the need to properly prime and paint the surface soon after the rust removal. The rust removal process opens up the bare metal surface and it will re-rust quickly in the presence of any moisture.
In consideration that you would be working on a part of your roof be prepared ahead of time how you plan to deal with matching in the paint with the reset of the roof. It could get to necessary to repaint a larger area or whole side of the roof.
For something the size of your average metal roof manully grinding the rust off by hand seems like an absolute last resort to me. Though probably a good solution for small stubborn patches. But for the time and effort it would save I would opt for a chemical solution any day, even if it was only 80% as effective. Besides, most metal roofs are fabricated with corrugations, ridges and dips, they're rarely flat. So I think you'd have a very tough time trying to remove the rust mechanically.
IME, Don't waste your time and money with naval jelly. BTDT, it mostly moves money from you to the manufacturer.
For a few 10's of dollars you can pick up an arbor and matching "surface conditioning discs" (essentially scotchbrite(tm) in a rotary mount) - if you have much to do, it's worthwhile to go there. If there's a huge amount, you can get belts (again, effectively scotchbrite(tm) in belt form) for a belt sander.
If the area is really small, simply getting a few hand pads of the coarse grade (brown) or medium (maroon) scotchbrite from a metalworking supplier might be enough. The green sold for most household uses is a bit wimpy/fine for this job. The conformability of the material makes it more effective than sandpaper for the most part.
Treating immediately after mechanical rust removal with a "rust conversion" primer is essential. You should also start putting money in the new roof fund - once a roof starts rusting, the clock is ticking.