I has a small rust spot on the rear top tube cable guide that I cleaned up and painted last spring. The rust has re-appeared. I am using Testor's model piant to touch up the spot because its a great match although perhaps not the most durable paint. Question: Is there a primer I could use which would offer better rust protection to avoid another re-occurence? Other tips?

You said rust, so I assume it's a steel frame. Since you need such a small amount, I doubt it would be cost effective, unless you find someone with some.

I've used Rustoleum primer with good results.

You could use self etching primer, or even a new stuff called POR (paint over rust)
However,,, neither option would be cost effective of a small spot

We use a product called Vactan on the submarine.

I has a small rust spot on the rear top tube cable guide that I cleaned up and painted last spring. The rust has re-appeared. I am using Testor's model piant to touch up the spot because its a great match although perhaps not the most durable paint. Question: Is there a primer I could use which would offer better rust protection to avoid another re-occurence? Other tips?

POR-15

I always use nail polish on chips. Never had any issues.