get some undercoating and spray it on
sand down or steel wool as much of the oxidation off as you can then undercoat the spot until you can get it professionally undercoated.
I spray likely spots for rust with spray oil like wd40 now and then (like my trunk lid, and under the rubber lining for the trunk (definate rust spots will show there).
i have a spot forming on my fender...it's just bubbling the paint a little and has some surface rust, but it bothers me alot
i hope i wont have to repaint just yet
for now i'm sanding it down, and then using touchup paint to protect it
clean the area well with soap and water first!!
'94 Supercharged E36 325is (150,000 original km's)
-Vortech V9 G-Trim @ ~10psi w/ BOV
-255LPH Walbro fuel pump
-TEIN SS Coilovers w/ Pillowball mounts
-Keskin KT1 17x8.5 & 17x10
-Remus full catback
-Authentic Rieger Infinity I body kit
-SMT6 EMS (Untuned)