1st rule of working on guns:
"When something doesn't unscrew with normal pressure, always soak the parts."
The best possible test proven soaking solution is 50/50 ATF (tranny fluid) and acetone. Twice as good as Kroil even though Kroil is test proven to be the best "off the shelf product".
As shown below, the scientific test proven top solution is not an off the shelf rust or corrosion product, and in fact exceeds them all. And off the shelf products are far from equally effective. Kano Kroil tops them all in testing and some are just a flat waste of money.
They are all inferior to (well down the list in test results as can be seen below), and more expensive than, ATF, plain old tranny fluid. None of the others have or will ever have the years and level of research or design to keep spotless and sustain the life of a $2500-$3000 and higher, piece of high tech equipment; your automatic transmission! It's also the least expensive product.
And if you have two parts corroded together solid and want them apart, the only thing better than ATF, again as test proven, is a 50/50 solution of ATF and acetone. Nothing approaches its efficacy and performance, nothing. Just soak and agitate, period!
Just a few representative quoted test results:
Average load (Lbs) to release a corroded test bolt and Price per fluid ounce
W/O penetrant.... .516 pounds $0.00
WD-40 .................238 pounds .. $0.25
PB Blaster ........... 214 pounds .. $0.35
Liquid Wrench ......127 pounds .. $0.21
Kano Kroil ............106 pounds .. $0.75
ATF-Acetone mix....53 pounds .. $0.10
(note: 53 lbs IIRC, was the load required to release the pre-corroded testing device.)
SOURCE: April/May 2007 edition of MACHINIST'S WORKSHOP scientific test of penetrating products to remove rust and measure the force required to loosen rusted-solid test devices.