Here is where I spew a ton of crap about penetrating oils! <grin>
WD-40 and the like will penetrate, but Liquid Wrench, Cyclo Breakaway, PB Blaster, and the like are all catalytic types which will actually creep into solid rust a lot better. My fave right now is the PB, not because I think it works better, but I like the smell better than the Breakaway!
I use Kroil whenever I can get it, and just apply it with a cheap brush out of a soup can. Kroil is pretty good, but IMO it's really nothing more than Lestoil and tranny fluid. I mixed up some of this once and it works actually quite well. I have used Lestoil for years to free frozen motors and so on. Weird stuff, but it works. I usually go in equal parts Lestoil, transmission fluid, and kerosene. This mixture will really do a great job of cleaning out your motor if you need to flush out the sludge.
You could add some WD40 to the mix if you like but kerosene is not far from the viscosity and lubricating qualities of WD40 anyway.
I'm surprised that Marvel Mystery Oil hasnt been mentioned yet. In my opinion this is merely good-smelling, and dyed red, kerosene. Oh while I am on a kerosene kick here, Diesel fuel will work just as well too.
Look on a can of Marvel. It reads there are many different ways to use it. Know what? You can use kerosene with all the same effects. Why I have seen my father hold the throttle open on an engine and slowly pour a pint of kero in the carb. It smokes like hell, but cleans and removes varnish deposits, and unsticks floats, etc, just like Marvel. In the end, the can of whatever you buy will be spent, and you'll have to get another. Just get whatever is cheapest, and above all, patience is key.
Goosing a rusted part up real nice with penetrant and then letting it sit awhile will make it easier to get it off. The stuff really dont work awefully fast.
Well thanks for reading all my rambling crap. As a professional mech, one tends to see almost everythign over the years. Hope some of this will help you.
Ol' Boggy Bottom
'83 & '84CJs, '51, '53, and '57 Willys Pickups