Sand blasting is usuallt the best option but it makes a mess.

if it was outside...I would say acid wash ....and yes patricia is right....sand blasting would make a compete mess... They would have to have a colour match with the paint....I would go to a good paint store and they could help ...of not go to the library and research your sort of home...and if all else fails....go to the local council....or govt dept that you pay for rates etc...not sure where that is as I am in Australia...good luck ...Robyn