Is resurfacing the brake pads in disc brakes (hydraulic or mechanical) necessary? If so, what would you use to do this - medium sandpaper?
I spoke to one mechanic while shopping for a bike, who suggested resurfacing on a monthly basis...
You've met a mechanic that either wants you to buy more brake pads from his/her shop or doesn't know what he/she is doing. There's really no good reason to resurface your disc brake pads, let alone on a monthly basis. All that's doing is decreasing the life of your pads and temporarily decreasing your braking power until your pads bed back into the rotors.
I think that the reason some mechanics feel the need to resurface disc brake pads is a carryover from the need to resurface rim brakes, which do need to be resurfaced in some instances.
I've had recent experience of glazed Shimano resin XT pads. I can't really tell how the pads got in that state as my use of the bike is pretty standard. I sanded the surface with medium grade paper and rebedded the pads which has restored them well.
I think it's only worth doing if you notice a loss in braking power and feel. Otherwise you are just wasting pad material and needing to bed the pads each time.