In a former life, I was a car valeter so:

Dont use paint stripper

Dont use abrasive pastes as the it will create thin patches of paintwork

Use thinners such as Finnigan's, a cotton cloth and gentle rubbing action. This will break down the foreign paint nicely. It will als remove tar spots and other sticky stuff.

Too much rubbing will mark the paint work!

Its hard work but worth it. Once clean, T-cut & polish and you will never know the difference.