This +1Save the WD-40 for dewatering things.
First, order a hand impact driver in the mail. About $20 for a brand name, nothing that might fly apart on you. And safety goggles if you don't own them. And a dead hammer or maul.
Use PBlaster per the directions, repeat 2x-3x.
Now use the impact driver on the screws. First whack can be set to "tighten", but make Real Damned Sure it is set to "Loosen" after the first whack. They will come out.
And anything except an impact driver, will chew up old slotted screw heads when you have to work them that hard. It is a tool you will rarely use, but you'll be very happy to have it when you need it. Keep the goggles with it, flying metal splinters can ruin your day if they do happen.
WD40 is not a lubricant or penetrating compound, it is a water displacement chemical. Save it for cleaning duty. Use Kano, Liquid Wrench, or PB Blaster in that order. You cam use auto transmission fluid as well.
Manual impact drivers are great and no boat or British car should be without one! $15 and it will last a lifetime.
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