Well, ofcourse this rusting is different from that found on a car.
First thing I would do is take the machine completly apart. Then you have 2 seperate problems.
The case which can be restored using sanding paper and zync spray or whatever it is called. First you sand off all the rust. Then you spray all the metal with the zync spray. It will get a grey coat. Then you have to put the case to gether. replace all the nails you removed and that sort of work.
As for the PCB that is the more serious problem. If it still operates it will be a cleaning job. We got special sprays at work. I have no idea what they are called but you spray the PCB with it causing it to foam.
Then you leave the sprayed PCB like that for awhile. After that you can use a brush and a piece of cloth to get rid of loosened dirt. (Make sure this is all ESD safe)
Inspect the cleaned PCB under a magnifying glass. When you see somthing suspicious and cannot see it clear using the magnifying glass use a microscope. The latter should not be needed I think. The A4000 has not that fine pitched components.
When you get a clearer view of the suspicious part of the PCB you can decide if you need to resolder.
When everything is nice and clean again you can resocket chips/simms if any and test the PCB.
After you find it works ok you put it back in the case.
Thats about how I would do it. Note: I'm not a professional hardware designer or tester. I'm an embedded softeware designer. When in testphase I assist collegues of mine finding problems in hardware. Thats how I gained some hardware knowlegde. My rust knowledge comes primairaly from my mexican build beetle that is the most rust happy car after the citroën 2cv
