Try a dead blow hammer first. Rubber exterior, lead shot interior. It works well.
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Penetrating Oils Machinist's Workshop MagT recently published some information on various penetrating oils. The magazine reports they tested penetrates for break out torque on rusted nuts. They are below, as forwarded by an ex-student and professional machinist. They arranged a subjective test of all the popular penetrates with the control being the torque required to remove the nut from a "scientifically rusted" environment
. *Penetrating oils ........... Average torque load to loosen*
No Oil used ................... 516 pounds
WD-40 ..................... ... 238 pounds
PB Blaster .................... 214 pounds
Liquid Wrench ...............127 pounds
Kano Kroil .................... 106 pounds
ATF*-Acetone mix............53 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 about 20% of the price. Steve from Godwin-Singer says that ATF-Acetone mix is best, but you can also use ATF and lacquer thinner in a 50-50 mix. *ATF=Automatic Transmission Fluid
As a side note, I have been using this mixture for a couple of years now, and have found that the Acetone does not dissolve with the ATF, so you have to keep shaking. Whereas lacquer thinner does mix, so I have been using a mix of 50-25-25. In storage keep the mixture sealed otherwise you lose the acetone and lacquer. This actually works well, power-steering fluid also works as well as trans fluid!