[Windflier]

Another vote for Kroil. I find that if Kroil won’t get something unstuck, the blue “wrench” is the only remaining option.

I'll definitely look for the Kroil at the auto parts store. What is the "blue wrench"?

When you say, "give the fastener a gentle tap", do you mean just tap it on the head with a hammer, or do you mean, give the wrench a gentle tap while it's inserted into the screw head?

Thanks for your help.

[Trod Upon]

Blue wrench is just slang for a torch flame. We don’t want to go there if possible :) Just give a little rap on the fastener head, like driving a nail. The object being to break the corrosion up slightly—really just kind of sending a shockwave through the assembly without deforming anything other than the corrosion. Do it after letting the Kroil work on it for a while If the fastener head is soft, like aluminum, take a little care not to mar it up. Just a sharp/quick, but still soft tap with the smallest ball peen you have. Good luck.