Ted's right of course Eldrick but it obviously depends how bad the problem is and how fussy you are. My 'new to me' 550 F1 had a few rusty spots around welds here and there, plus a few chips - not surprising - it's over 47 years old! It's to be a tidy looking user - not concours. I scrape the areas with a scalpel, de- rust if needed with Jenolite and touch in with Rustins Gloss Black using a small brush on the bigger areas and varying sizes of copper wire, dipped in the paint for the small chips. It looks a bit grey when it first goes on but dries to a pretty good match (in my case!). I did my 350/4 the same way nigh on 20 years ago and it's still good.
Ian