What to do about this rust?

[In The Ten Ring]

I am about to reassemble my front end but I have some rust there yet....I'm not able to do a frame off and I cannot do my own media blasting or even painting.

I was just going to reassemble my newly painted replacement parts (all NOS or OEM) but should I get rid of this old rust first?

This is the driver's side front fender, it's good overall but has some rust where parts meet. What should I do? How should I do it?