I'm trying to convert an old Victorian screen door into a trellis. The
hinge screws are rusted in, got one out and the other one is totally
stuck, three more to go on the other hinge. I sprayed it with WD40, got
some on the wood, and am now worried that the primer coat won't bond
properly.

Is there any way I can treat it without buying another can of something?

Also, how do carpenters get old rusted screws out? I am using the right
size screwdriver. The hinges are really neat old things, but I don't
want to leave them on. They are decorative and spring loaded on the
inside, but are just some metal, so I'd kind of like to get them off
without ruining them if I can, like if I have to somehow cut them, I can
twist the screw out with a pair of pliers.