+1 plusgas... It does the business.
I will add though. what a lot of DIY'rs tend do do is soak bolts and nuts in WD40, or any type of penetration oil... Thats ok.... But take it from an old school time served mechanic, it can also defeat the object!
You have to remember that penetrating oil is: oil and it has slipping properties when it comes to fitting a torx bit, alan key or even a spanner or socket on the offending nut or bolt, this often causes the item you are trying to free off to castle or round off as the tool is harder than the nut/bolt... The oil only adds to the problem making it easier to do
An old trick is to:
Remove any excess rust and deposits with a wire brush.
with your tool off choice for the nut/bolt, give a tap on it with a hammer towards the direction of the bolt downwards to break the obligitary corrosion seal between the two parts
Soak the nut/ bolt, leave it for 15 minutes , it will have done its job by then.
Now the part that no one thinks of: Use brake and clutch cleaner spray to remove the excess penetration oil and offers a nice, dry, oil free surface for the tool to grip onto.
Last part: Don't try to free off at first, actually try to nip the nut/bolt up further just even 1/8th of a turn... Now loosen as normal and apply apply even, firm pressure to your chosen tool without yanking. With allen/torx keys, try using a long bar over the end for more leverage (room permitting) failing that use a ring spanner over the the allan key and apply pressure gently.
Another handy tip when it comes to freeing of damaged or really tight nuts... If it still wont budge. Get hold of a new nut with the same size and thread, aplly a couple of tiny drops of superglue to one face of the new nut and wind the nut down the thread with the glued face down towards the old nut and nip it onto the old nut, matching the flat sides... Leave it to dry for ten minutes and then apply the spanner/ socket over both nuts... It should come out a lot better. Super glue is quite useless when it comes to hard plastics etc. But it grips metal to metal very well... Just be careful not to get any on the thread part.
Removing nylock nuts? Gently heat the nut with a heat gun and the plastic will start to soften... Don't go too far or it will melt all over the thread... As quick as you can, apply the tool to loosen off the nut as the plastic starts to harden slowly and cool down... It will spin off easily the quicker you are at doing it.
Failing all of the above... A heat gun applied directly on the nut /bolt head only followed by a quick tap off a hammer, then loosen off asap before it cools will always do the trick
Hope this helps