[Trod Upon]

Another vote for Kroil. I find that if Kroil won’t get something unstuck, the blue “wrench” is the only remaining option. Let it sit a while and give the fastener a gentle tap with something before you try to budge it. Steady pressure seems to get it moving eventually.

[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.