Many times metal cures are indeed a double edge sword. It can bring both protection and danger. For example a metal sword can serve as protection but if place haphazardly around children it can serve to be quite dangerous. Gold and silver can show great wealth and attract admirers but it also attracts burglars and robbers.
Even though this is the case one can still use metal to enhance luck of helpful people in the northwest, boost creativity and child bearing in the west and increase ones career prospects in the north. The best is to use non precious metal which is round and is non threatening in shape.
The best time to cure with metal is when the 5 yellow or 2 black enters the flying star chart either in your home's natal chart or in the annual chart. It's like the surgeon's scalpel when a sick patient arrives. Metal can also clear space due to it's tonal qualities.
In Flying Star Feng Shui, it is said that soft metals like brass or those of gold colors actually enhances the 2 black from unlucky to balanced harmony. Whereas using hard metals, to cure the 2 black, such as bronze is more fortunate in fire sectors.