I was told the following by an 80 year old mechanic who's retired. It's NOT suggested for use on aluminum, but it worked like an absolute charm on my brother's pickup.
He has an f-350, and the exhaust manifold studs were so ridiculously frozen in the heads, nothing we did could get them out. Mind you, between my dad, my brother, and myself, we have a combined total of at least 55yrs dealing with crappy cars and rusted bolts/nuts.
This trick worked like an absolute charm, and is invaluable for a situation like this, but only as a last resort, because 90% of the time, you won't have much trouble.
Heat the nut/bolt up until it's red hot. Grab a candle, any candle will do. Once the bolt/nut is red hot, melt the candle into the threads. Keep melting it in until you feel it's sufficient.
Now wait for it to cool. Grab any wrench, and the bolt/nut will turn right off. We did this to 5 stripped studs on my brother's truck. It was incredible, and worked flawlessly every time.
According to the oldschool mechanic, the wax seeps into the threads when the nut/bolt is red hot. (you must melt quite a bit of the candle into it, otherwise it just burns up) When the assembly cools, the wax forms a slick barrier between the nut/bolt and the threads. It takes almost no effort to remove it from there on out.
I had my doubts, but it worked like charm!

Give it a try when you get the chance!