Penatrating oils

Try a specific oil like 3-IN-ONE® Fast-Acting Penetrant
Patience, may best to wait overnight or days to prevent over powering, breaking a bolt.

There are many penetrating oils
breaks apart tough bonds, and is better than the competition in preventing new rust from forming. 3-IN-ONE Fast-Acting Penetrant is safe to use on metal, plastic, and rubber, and the twist-tip Marksman Spout ensures precise, no-mess and leak-free application, so you can get unstuck easily without all the mess.USE FORFreeing stuck and rusted bolts, nuts, hinges, threads, and chains
Preventing rust on parts, tools, hunting, fishing, cycling and marine gearPENETRATES UP TO 3 TIMES FASTER THAN LEADING COMPETITORS*

1.File a flat on each side and grip the bolt with a pair of mole grips 2 Give the bolt a sharp tap with a hammer or punch set the punch at 45 degrees pointing outwards and give it a few taps.
3. try soaking the bolt with penatraing fluid see my comments below.

A 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,
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 - 50 automatic transmission fluid and acetone.
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.
Good Luck,
Crofter

Usually the torque spec for those nuts is pretty high, and they suffer from what I call "environmental torque" as well (exposure to the elements and to heat cycling that can cause it to seize). Your best bet first off to try, is to spray it down with PB Blaster several times, with maybe 15 minutes between sprays. This will keep it doused and allow it to penetrate. If you have some, or can get some, a penetrating oil like Deep Creep can work even better, as it can get down into gaps a millionth of an inch in size.

If that doesn't work, many tool rental places can rent you an electric impact gun for maybe $20/day or so. Something like that, which can give you an instantaneous blast of strong torque, can often break a stuck nut free. You can also put a flat-blade screwdriver against each flat side of the nut, one at a time, and whack the screwdriver handle with a hammer - sometimes this can break the seizing free.

At this point, you're down to chemical penetrants and brute force. I've found that one or more of these methods usually works for me.