Having followed the interesting molasses & vinegar thread, and tested Evaporust, I thought it would be interesting to compare the electrolytic method.
Google Search for Electrolytic Rust Removal.
I bought some washing soda, cheap as chips, dissolved about 1/2 cup in about 18 litres of water and arranged some anodes (sacrificial iron, so used some scrap tube). I suspended the other mirror from my Commer van from a piece of wood, and connected my automotive battery charger.
Two things to note at this stage: Unlike the other people who described their efforts on the web, my setup didn't immediately start making bubbles of hydrogen, and the meter on the charger didn't indicate any current.
Anyway, here's the before: (This can be directly compared to the Evaporust as it's the opposite mirror to the one in that test).
Basic automotive charger (4 amp):
After six hours (the solution was clear when I started):
I ended up leaving it overnight.
Results: - Note: The charger was now indicating a reasonable current, and the bubbles were obvious. You cannot let the gas accumulate as it's dangerous from an exploding point of view. Also, the part in the photos is wet. Once dried, it's clear to see there's no rust left. The stem of the mirror is as eroded as the other, hence the off scalloped look.
The solution now had a layer of gunk on the top. I rinsed off the part.
So it's about as effective as the Evaoprust, although it did soften and loosen the paint - potentially a good thing, potentially a bad thing. The solution is caustic, but not enough to mean you can get it on your skin for a while. You don't want to dip your hand in the solution while your charger is on, obviously, but the solution is fairly benign and can be tipped into the garden (on a par with Evaporust).
It is cheaper, in the region of 50 cents of materials and power vs about $10 for the Evaporust (consumed in the first test). The Evaporust is easier, as the rust just disappears and all you need to do is soak it. The electrolytic method is a bit more involved, as you need to hook up all the gear, rotate the part, and perhaps give it a bit of a wire brushing if the rust is as thick as this was. It's cheaper than vinegar or molasses, I would think.
Both Evaporust and this method are better than wire brushing or acid based methods, I think, although limited to the size of part you can get in a bucket.
