Paint or undercoat?

I just took my old rusted out gas tank of my roadrunner. I decided before I put another gas tank on I should clean the trunk floor up and try to keep it from rusting out. (it has no holes now) My question is after I wire brush it, should I paint it or undercoat it? I might also do the frame in that area while I'm at it. Thanks for any help.

koondawg
MOPAR OR NO CAR!