How long do these anti-corrosion coatings last then?
Every time you hit the brakes hard you're exposing a fresh metal surface no? I thought so long as you go for standard discs (i.e. not carbon ceramic or other exotic materials), surface rusting on the friction patch was pretty much unavoidable...
Correct. The surface of a steel disc is definitely going to corrode. The anti-corrosion coatings are only of use around the hubs.
What I do is to simply spray the hub part of the disc with some neutral-coloured, heat-resistant paint before fitting the new discs. No unsightly rusty hubs that way.
For best results with any disc and pad combination you should "bed in" the discs. Drive the first 300 or so miles with only careful, light braking. Then the next 500 to 1 000 miles with light to moderate (normal) braking, until you can see a uniform bluish colour over the entire braking surface of the disc.
Once you see that, you can do the proper bedding in. Get on some open, quiet piece of road. Do a bit of moderate braking just to get the discs warm. Then get up to 70 miles per hour and brake hard down to 20 or so miles per hour. Accelerate back up to 70 and brake hard again, repeating this process 6 or 7 times until you feel the brakes fade a little. You may smell the pads or even see a bit of smoke.
Then drive fast-ish for 15 - 20 minutes to give the discs time to cool down completely again. Do not, whatever you do, stop while the discs are hot and keep your foot on the brake, as this will deposit a clump of pad material on the discs in one spot.
If you do the bedding in correctly, there will now be some of the pad material embedded in the surface of the disc, increasing friction and making the disc last longer.