ncy-sud- As you're undoubtedly aware, you need to do something about the rust before you apply the touchup paint. There are mechanical and chemical solutions and I've had both work fine (and *not* work fine when I botched it up or used crappy products).
Mechanical- use a "rust pen" to abrade away the rust. Google "rust pen" to see what I'm talking about. There are big and small ones for different size chips. You gotta sharpen the filaments to fit the chip and avoid touching the surrounding paint.
That will probably get most, but not all, of the rust out.
OR...my preferred method- use a tiny diamond burr on a Dremel to grind the chip out. Use magnification if necessary so you can truly see what you're doing. But Dremels and diamond grinding bits cost a lot more than the Rust Pens.
Then use a chemical to treat the remaining rust. Google up "Rust Converter". Rust converters work better than mere "metal prep"/acid products, but those can be good too (of those, I prefer Rust Bullet's Metal Blast by a *HUGE* margin over other brands).
The rust converter will leave "stuff" that will act as a primer.
You can seal the chip with a "paint over rust" product to make sure the rust stays killed. POR-15 is what most people think of along these lines, but IME it's a pretty crappy product. I like Eastwood's Rust Encapsulator and Rust Bullet *MUCH* better.
Then apply your touchup paint. Note I didn't mention primer; I let the rust converter and/or the Rust Encapsulator/Bullet stuff do the priming.
Sorry I didn't have any links to the stuff I recommended, but you'll be able to find all that stuff online easily enough.
