This is a big issue in damp climates, it doesn't seem to be any problem at all in other climates.
My finding is that potting in polyester resin is a sure way to disaster, my suspicion is that moisture gets between the magnet and rotor disc and electrolytic corrosion starts. I also think the nickel plating is a flawed process and unless you can keep it perfect it really doesn't give much protection.
I had serious problems with polyester potted magnets and the corrosion was dreadful. One of those scrap magnets has sat outside now on a wall for several years, the nickel has now gone completely but the rust has not progressed much and it is not much worse than when I threw it out.
My conclusion is don't pot in polyester, probably don't pot at all but if you try it, use epoxy. I have a feeling that a thin coat of epoxy, which remains flexible is far more likely to succeed. I have tried POR 15 on one rotor and so far it is ok, I suspect this could be the best option.
If you have to use a glue to hold the magnets then make it epoxy. If you have some means of preventing the magnets from flying out the best method is to use POR 15 on the rotor plate, a film of POR 15 on the magnet surface that touches the plate( let totor plate dry before adding magnets). Finally a couple of good coats of Por 15. Possibly some form of flexible coating over that may be beneficial but as long as you don't damage it physically the POR 15 seems to be very durable.
I suspect yhat if you can absolutely prevent moisture from penetrating between magnet and steel plate there is no issue. In the real world this is most likely an impossibility so I suspect a non conduction barrier between magnet and steel is a wise thing to aim for. I also have a suspicion that epoxy coated magnets may be far preferable to the nickel.
My experience on the repaired rotors needs more time to evaluate but it seems as though the thin epoxy method and the POR15 method have dealyed the problem well even starting with damaged magnets. With new magnets and this treatment it looks as though you may get a sensible working life.
If you are in a reasonable climate, as long as you avoid the polyester potting, magnet life may not be a factor. In a damp climate with nickel plated magnets potted in polyester neo is not really viable, with a life of only about 3 years.
Flux