Go to an auto body paint supplier and ask if he has a sample of etching primer.

Wal-Mart has a rust remover, cheapo, in a white bottle. It works well with a Q-tip or toothbrush. Follow the directions. And remember all this is so much easier with a frame free of other parts.

If I use nail polish to touch up, I smooth it out and then mask off the section well and shoot some hobby clear acrylic over it, very little.

I've had good luck with Testor's, and also Duplicolor automotive touch-up paint (also at Wal-Mart and lots of other places). Burgundys, silver metallics, black and some whites, in particular. Good applicator makes me look more dextrous than I am.

And sometimes, like A. Winthrop and JunkYardBike have told me, sometimes, you let it ride and ride it.