I think the most effective tool is either an angle grinder with a cup-shaped wire brush (quite pricey; the wire brush costs as much as a cheap angle grinder, but it's worth it), or an electric drill with a paint-stripping disc. The wire brush is preferable for rust removal, the paint stripping disc is preferable for, umm, stripping paint. The stripping disc is made of coarse black fibres.
The wire brush looks like this:
Other, straight-wire types are not a good idea, as they may not cope with the 10,000+RPM of the angle grinder.
Wear safety glasses and earmuffs.
No point having a lifetime eye injury for the sake of a rust repair, and also no point in having ringing in your ears that night
Hand wire brushing is not sufficient in my experience (it just polishes up the surface of the rust), and a power drill with a wire brush is also too weak. It really needs to be a proper-sized wire brush on an angle grinder - even the cheapest angle grinder will suffice, it's amazing just how cheap they are - $15 or 6 pounds in your money! Everyone should have one, and you probably already do?
The sander might be powerful enough (sanders come in lots of different types - the random-orbital type with a square pad (palm sander) won't be enough, the rotary type might be) but won't cope with the pitted surface that rust usually creates. In other words, sandpaper will clean the rust off the top surface really well, but still leave rust in the pits. The wire brush gets in there properly.
You will probably find that the surface rust turns into holes. Don't worry - keep going, both sides if possible, until you are left with only shiny (pitted and holed!) steel. That way, once finished and painted, the rust won't continue.
It is really important to remove all the rust at this stage - if you don't, the rust will carry on under whatever you put on top. Because rust takes up more space than steel, it bubbles and splits the filler/paint, which lets water in, which feeds the rust...
Then you can use etch primer, or another type of rust-fighting primer. Then body filler (bog), fibreglass-reinforced if the holes are larger than a few millimetres. Apply on both sides of the panel for the most durable result. You can have great success; the repair can last for many years. Or, if there was rust still there, it can show through in a few months.
I know of at least one Uno door that didn't have much steel left in the bottom few centimetres of the outer skin, and was basically made of fibreglass-reinforced filler - yet even after several years, only the smallest rust bubble showed.
Sand the fibre-filler really well with coarse 80-grit sandpaper on a block, use another coat of fine filler to finish off, sand again with 80 grit and 240 grit, then spray a couple of coats of normal primer, sand that with 400 grit, and you're ready for paint. Most of the work is in the sanding. The flat surfaces and simple lines of the Uno are easy to work with - much easier than, say, a Morris Minor or a MkII Jaguar.
This advice applies only to cosmetic rust in such things as wheelarch edges or door bottoms. Widespread areas of rust in a removable panel, such as a door, mean you are better off to replace the panel. Other structural rust, at the edge of a floor for example, needs cutting out and a new plate welding in. It's not that expensive to get someone else to do that for you, especially if you finish it off yourself - welding equipment, on the other hand, is pricey and needs a skilled hand (I'm still no good at welding despite many attempts
)
-Alex