Concrete mud

I've noticed in a few out of the way spots that my vehicle has a build up of mud that has dried to the consistency of concrete. I have tried using various cleaners to soften the buildup, all to no avail.

The vehicle is second hand as has obviously been off road in the north west, so the mud obviously originated there.

Is there some sort of solvent/trick that will loosen this stuff so it can be cleaned off or do I continue to have to go around and chip it off with a sharp stick (file), and risk damaging the paint work?

I'm sure that if I just leave it there its probably going to end up causing rust. Its mostly in obsure places that you dont get to look at until you get the vehicle home and start fiddling with it.