John,
are you sure its not been taken back to metal, rubbed down and re-painted? sure you are 100% its just overspray...? as this method WILL remove the top coat !
The thinners will strip it bare if you leave it on, but apply a small amount to a rag and wipe softly to start, the rag will change to the colour being removed but wipe over with panel wipe to remove ALL traces of thinners after.
try it with a finger 1st on the underside,you will soon work out how much pressure is required, easy job but, just take your time ! the old paint will gleam through.
If you are unsure then go for the soft option G3 or G10 cutting paste, very much like t-cut but with more grit/cutting G1 being HARSH G10 FINE.
kipper.