Auto body repair & detailing: welding, rust patches, arc welders

Question
What is the proper type of weld to use for repairing body rot.  I have several areas 3-4 inches at their widest point and want to repair them.  I see arc welders can be purchased fairly inexpensively but don't know if this is the correct type of welder.  

Answer
We use wire feed welders, but you weld sheet metal patches over the rust or cut out the rust and then weld a patch.  So I suppose you could use any welder that you could do spot welds with. As long as it don't burn to hot of a weld or it has a variable heat setting. Now a days we use 3M Panel Bond for gluing rust patches on. I know it sounds crazy but it works great and it has rust prohibitors in it.
           Thank you for the question !
           Ken