chook93

Well that is a very open ended question. The metallic parts of which a large percentage are usually aluminium are typically very strong and stiff. But when they are bent generally can not be "hammered" back into shape as this will cause a weak point and you can get stress cracking. The skins are generally easy to replace. But if they are damaged, can either have the damage cut out with a patch repair to the area or a complete replacement of the part. It is rare for only the skin to be damaged though as there is a lot of structure holding the aircraft together, so there is usually damage to the underlying structure also, if there has been an incident with a truck or other aircraft. 
 
If it is only minor like say a small scratch the scratch can often be blended out up to say 20% of the thickness of the part as usually aircraft parts are designed with at least a 1.5 times designed load limit, built into the part 
 
The applicable aircraft's repair manual will usually have repairs for damage to structure also in which case the damage is cut out with a new piece inserted and a repair doubler to hold it altogether and return the structural integrity of the part or again replacement of the part, depending on the location of the damaged part,size,cost, time taken and structural load of the part etc are all considered. 
 
There are typically 3 class's of structure on an aircraft. Primary, secondary and tertiary. primary and secondary are both load bearing and critical to the structural integrity of the aircraft. Tertiary is fairings and the like that if damaged will not affect the structural integrity of the aircraft
 
If the damage is very bad and outside of the aircraft's repair manual an aircraft engineer will design a one off repair especially for the part, for then the structural and mechanical tradesmen to carry out.
 
Note that some parts are made by hammering them into shape for the many different shapes and curves of the aircraft but these are made from aluminium that is soft and once the desired shape or curve has been made the part is then heat treated to a high temperature which strengthens the aluminium and then be ready to fit to the aircraft. This unfortunately is happening less and less as more and more composite aircraft are coming out. It is a hard skill to master but is really awesome to see some of the shapes on parts that can be made
 
If it is composite it can vary from simply adding some resin or adhesive to the damaged part to cutting out the damage usually in steps as you cut through each ply of the skin and then if the honeycomb core is damaged cutting it out and replacing it with new honeycomb then laying new plies on each layer of the skin as it was step cut, all in the same orientation as each original ply or layer. Usually with a few extra plies over the entire repair tying the repair into the original part returning the strength to the part.
 
For me it is disappointing to see all these new composite aircraft. Composite repairs are usually very time consuming boring and monotonous, and although you have to be a highly trained person to carry out the repairs, i feel there is actually not much skill in the process. Compared to repairing an aluminium part where especially if it is a curved part requires much more skill in turning a flat sheet of metal into an intricate shape. Aluminium is also much nicer to work with than composite and for me tend to get a higher job satisfaction. 
 
But you can not deny the strength and weight characteristics of composite generally are superior than aluminum 
 
Hope that clears up some things for you. :rolleyes:
 
Regards
 
Hayden