[Ascaris]

Having spent last winter doing the same thing, my advice would be if it is just surface rust without major pitting, you can grind down to fresh metal and / or treat with a rust stopper, then refinish.

If it is much worse than that, go for surgery. And when you start cutting, cut it ALL out. If you are going to the trouble of welding in a new patch, the time and effort isn't in the welding. It is in the prep and finish work which takes the same amount of time for a small patch as a large one.

Good luck