My S2 (came from Japan) looked really good when I bought it but after 4 years the oxidation came through with a vengence - mostly engine cover (bonnet?) and some around the rear hatch, rear flares and top of the bumpers, etc. This first showed as a little bit of shadow under the shine and then over the winter break - it just blossomed through the paint.
Took the car to a detailer for power polish - looked good when I got it back - drove it a couple of times, otherwise parked in the garage. After about 1 month I washed it and all the oxidation was back.
I tried "cleaner" polish, clay bars and all sorts and resorted to rubbing compound to finally get most of it off and back to the detailer for re-do. Much heated discussion about what should have been done the first time, etc. (Their opinion was it was shiny when it left and the fact that it only lasted until the car was washed was my problem - not theirs). Its OK now but I know the oxidation will come back as you can see areas under the shine if you get the light just right. Also - some marks still in the paint from the compound even though I applied it by hand.
Anyway - the end result was I should have tried a National chain of detailers that have a good reputation and guarantee their work (or at least are honest about the results you can expect before you spend the money) or more likely - I should have color sanded it (a great idea if you haven't polished off most of the paint over the years).
P.S. the National chain of detailers quoted C$450 for the job - the useless weasels did it for C$285 - all I can say for sure is that the C$285 was wasted.....