you know brake fluid is a great penetrant too it will eat/seep it's way through all kinds of stuff.
it's a last resort, and needs cleaned up afterwards, but it works on the super rusty stuff.
I'm not pouring any in my gun solvent,, but on a stuck bleeder valve or rear spring bolts yeah.
that and a shot of the old Mopar exhaust heat valve unsticker stuff works wonders.
if it's something fine and delicate your not gonna get any better than acetone and ATF mixed 50-50, it will get down into fine threads and small crevices and stay there better than anything else.
My Granddad was maintenence foreman for a cement plant from the late 40s to the late 60s. Brake Fluid was his go to penetrant. I'm sure there's been advances since that time, but he used alot of the stuff.