'Chuggs' has several rust spots which can go deep. I use a 'dremel' to grind all the rust out and then tap the remaining steel to see how thin it is - if a gentle tap causes distortion I will cut it out. Yesterday a new patch ground out nicely and has left a reduced section which looks perfect for a bit of leading. Is this a good thing to do or am I storing up a heap of trouble? Below is a less clear case...
IMO cut out and weld if you want to do a proper job. The metal on these buses is thin as it is, without pits from rust like that. I sometimes use a power file and plunge it through the rust spot, if you know what I mean. This creates a small square hole surrounded by decent steel. Then weld a little square of fresh steel in, grind welds flat and prime.
