when to walk away?
To some extent it depends on how badly corroded but you have several options. First of all, you can buy an extrusion pretty cheaply through Dwyer or Kenyon (both are on line). Trace the spar section on a piece of paper, they will have an easy time recognizing their own. It is not that hard to move everything from your old spar to a new one. Plan ''B'' is to sleave the mast. Fir works well for sleaving. To sleave the mast you shape a plug out of piece of wood carving it to the shape of the inside of the spar. Coat the ends of the wood with epoxy to seal the wood and when it cures slip the plug into the mast to provide extra support to the damaged section.