How to remove rust from desktop computer cabinet case
The original metal finish inside a PC it typically a type of zinc plating. no paint will exactly match it.

1: I don't know of any chemical that will remove rust without also attacking the plating. fish oil will stop the rust, but it take a long time to dry.

2: galvanised iron primer

3: take it to a paint shop, they'll probably be able to match the finish better than anyone can guess over the internet.

Consider going to a new finish, using automotive paints, you get a custom look at about the same expense as a repair.

It rusts because the zinc coating is applied to the sheet metal before the case is assembled, so all the cut edges have no plating. also something must be getting it wet, perhaps something causes condensation?