Had the same marks on the stock exhaust on my 325. Just used to remove them, degrease them and use autosol...never really did the job though I'm afraid
Had the same as Donal. In the end I just bought new tips as the old ones were pitted quite badly
Current:
'06 530d Silver Grey, '04 325Ci Orient Blau
Previous:
'03 M3 Silver Grey
'02 325Ci Imola Red II
'98 318iS Cosmos Schwartz
Looks like the coating has cracked. If new ones are prohibitively dear then I know carrig do a choking process and I was quite impressed with it when I saw it!
Sorry the auto correct got me. Yep carfit and that should say chroming not choking.
If ever there was a perfect excuse to treat the beast to an eisenmann exhaust... this is it You couldn't possibly put up with that minor blemish Mark!