Do you mean stuff that presumably hasn't been buffed off (which I've never seen, but don't work in a bodyshop, so wont), or do you mean the dust it creates that often gets left behind in panel gaps?!
or...
Are you talking about dried on lines that get stuck on the edges of protection films etc? I see this alot, where the owner has been over keen with wax etc and it builds on the stone chip protection films behind wheels and covering certain panels. If it's this one, then it's a good question - I use a cotton wool bud with IPA (isopropanol) on, which removes most of it and certainly softens it up, then a toothpick used with care will remove the rest