This is what you really want:
http://www.wivco.com/shake_n_break.htm
be sure to set your air pressure at 30 psi, no more.
You need an air hammer to use this but they are not expensive.
It is the vibration that really loosens the rust.
There are also youtubes of how to use it, as simple as it is.
never let rusted screws intimidate you again....