I'm looking to buy a a magnet which can detect drywall screws in the wall studs and ceilings joists. The screws will be covered over by drywall joint compound and drywall tape. Could someone suggest a very strong magnet which could easily pickup the screws which are covered over by tape and drywall joint compound. I've heard of rare-earth or neodymium magnet magnets. Any help would be appreciated ?