The light sanding may help , it gives the epoxy something to grip onto , and is the usual advice for getting 5min epoxy to stick to metal, JB Weld might work too but I think it has metal powder in it too ? so not sure if that would effect the magnetic field or not?
100% silicon should work too , but you'll need to wait overnight for it to cure before moving anything.... it sticks to Chrome/Ali etc at work so should work for that too...
