I assume you are talking about stains from bird****/sap that has been removed and has left etched marks into the finish {erosion} as compared to laying on top of the paint?
Etching from acids in rain/birdpoop/sap/battery acid/nuclear fallout/etc is very difficult to remove without compromising the remainder of the clearcoat as you basically have to remove {compound/wet sand/polish} away enough of the top clear coat to get down below past the depth of the stain....some stains may be shallow, others deep enough to require so much paint removal, there is nothing left. Wash the car and swing by a local profesional detailer who can examine your paint problems and give you expert advice on the possibilities of partial or total elimination of the problem areas. They will probobly start with a device to measure your current paint thickness. Go to
http://www.autopia.org for a good carcare forum!