External patches are normal, although they may not be used in some specific aerodynamically critical areas.
In the business they are called "scab patches" and most are the result of baggage and catering trucks striking the aircraft. Damage is repaired in accordance with a "Structure Repairs Manual" or SRM and, where damage exceeds the SRM limits, a special repair scheme may have to be produced - usually in consultation with the aircraft manufacturer.
External patch repairs are generally removed and replaced with a flush repair during heavy maintenance visits. Whenever a repair is done, the damage is recorded on a damage record chart, which is one of the records which stay with each aircraft for life.