Soak, heat, beat, repeat!
Here's the 'study' that always gets referenced in these discussions.
"Machinist's Workshop" recently published information on various penetrating oils. The magazine reports they tested these products for "break out "torque" on rusted nuts and bolts. A subjective test was made of popular penetrating oils, with the unit of merit being the torque required to remove the nut from a "scientifically rusted" bolt. Average torque load to loosen nut: No Oil used ........................516 foot pounds WD-40 ..................... ........238 foot pounds PB Blaster .........................214 foot pounds Liquid Wrench ......................127 foot pounds Kano Kroil .........................106 foot pounds ATF/Acetone mix...................... 53 foot pounds The ATF/Acetone mix is a "home brew" mix of 50/50 automatic transmission fluid and acetone. Note this "home brew" released bolts better than any commercial product in this one particular test. Our local machinist group mixed up a batch, and we all now use it with equally good results. Note also that Liquid Wrench is almost as good as Kroil for 20% of the price. ATF/Acetone mix is best, but you can also use ATF and lacquer thinner in a 50/50 mix. ATF = Any type of Automatic Transmission Fluid"