WD40 is an inferior rust penetrating lube. Use PB Blaster or Aerokroil for better results. Never use WD40 on hinges, latches or any other moving metal part because it dissolves grease. Use silicone grease or lithium spray grease instead.
There you have it from the Zetec experts, remove the plugs and replace. Now you might have a problem re-installing the plugs and getting it to seal correctly. I can't think of any industrial compounds to use to seal the threads. I don't think thread lock would be a good idea, and I'd worry that thread sealant would last in that environment. The only thing that I can think to use would be Permatex Indian head gasket tack which should be available on the same aisle as RTV in any parts store. It's in a brown bottle about 2" tall. Paint that on the plug threads before you screw it back in. Finding a replacement plug could prove very difficult, so I hope you don't have to damage that one to remove it like 1Turbofocus wrote.
Another point to note, since you're new to repairs and such on vehicles. While you're there getting some gasket tack, get you some anti-seize for your spark plugs as well. It should always be used on spark plugs in aluminum heads, and I also use it on outer CV shafts. You can also find anti-seize at hardware stores, but make sure that you purchase the nickel based type, sometimes labeled as "Nuclear grade", which is good for 2400F. Copper based is only good for 500F, and not suitable for spark plugs. Blue thread lock is good to have around, but you have no need for red. The difference is that blue can be removed with extra torque, but red must be heated with a propane torch to be removed. You also have no uses for high temp copper RTV. Blue (coolant), black (oil), and grey (both) are all you'll ever need. Gray is the most effective/long lasting from my experiences, but has extremely long dry and cure times, so not always the best idea for driveway repairs.
You could use blue RTV on the plug threads as well if you can't find gasket tack.
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