Copied from other forum but may be of use to someone in your position.
“Machinist’s Workshop magazine actually tested penetrants for break out torque on rusted nuts. Significant results! They are below, as forwarded by an ex-student and professional machinist, Bud Baker.
*Don’t forget the April 2007 “Machinist’s Workshop” magazine comparison
test.*
**
*They arranged a subjective test of all the popular penetrants with
the control being the torque required to remove the nut from a
“scientifically rusted” environment.*
**
*Penetrating oil ….. Average load*
None ………………… 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 was a “home brew” mix of 50% acetone – 50% automatic transmission fluid.*
*Note the “home brew” was 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 about
as good as “Kroil” for about 20% of the price. * “
So maybe some of that but failing the above get seatpost in a bench vice and use frame as leverage.
HTH