How long does it take for the rust to come through the paint?

If there are little streaks of rust (not flakes, just brownish area) left on the sheet metal and painted over, how well it'll last? Will the rust come through the paint and how long will it take? I assume that under the paint there is no air or moisture for rust to develop.

I was also wondering, if there is any dirt or grease left on the body of the tractor, how does the paint behave then?

I'm not asking because I'm too lazy to do the work properly, but there is always some rust left in places where you can't get with sand paper. I'm also interested in the subject in general, because I don't know much about painting. And maybe there is a remote possibility that I will get lazy...

Use a rust converter over the exposed rust. I use the kind that forms a black, thick film after 24 hours on the surface. This seems to work fine. just prime and paint as usual after that.

As far as oil and grease..they have to go if you have even a modicum of pride in your work. These contaminates and paint do not go together in any form or fashion. You are asking for trouble as the paint will flake off in a day or two leaving behind uncovered oil, grease and old paint. A high pressure air hose and kerosene will be better than nothing at all in that area.

imho