before doing anything , mask the non-rusted paint area with 2-3 layers of painter masking tape..
using 150 grid dry sand paper clean until you reach complete clean bare metal .. inside out (all inner outer sides)(you will need a mirror)
(if you cant clean the rust completely or there is a hole - I'm afraid thats a body shop work)
clean the tapes..
now mask the area 0.5 cms bigger from all directions
now sand the area with 400 overlapping to the good paint (0.5 cms bigger from every direction)
spray (6-7 layers- use hair dryer between every 2 layers) standox/glassurite epoxy (with hardener and celulosic thinner)designed for cars.. (1.4 or smaller nozzle size for best results)
wait 24-36 hours depending on temperature (do not drive car)
sand with 800 wet , use hair dryer again (be sure it become dry completely)
increase the sanding area size 2-3 cms bigger in all directions (and arrange the mask area accordingly)
prepare your color mixture in a paintshop (250 gr will be more than enough you will use the excess for touch up)
check the color tone on a test card and compare with car
+ clearcoat with hardener
apply paint and clear coat respectively overlapping the clean area..
be warned for best results fine size hvlp guns are expensive..
after drying sand with 2000 and polish..
ps: dont know the prices there but for those kind of small area jobs, small size (half size) hvlp guns are much better..
pps: minimize air pressure, paint shape must be oval and make a test paint on a metal surface before actual painting.. and clean the gun immediately after painting..