I don't think Kroil should even be classified as a penetrating oil because it has failed for me so many times. If Kroil loosens stuck threads, they probably weren't stuck all that bad in the first place. I have lots of it, and I use it as an assembly lube for rusty muffler clamps and things like that.
For really stuck threads, I agree with the repeated soldering iron heating and applications of a really GOOD penetrating oil as Mike suggested. Whenever I see electric soldering irons at automotive swap meets or flea markets for a buck or two, I buy them. I'm talking about the straight type with larger copper tips, usually 40 to 100 watts. I grind the tip to fit closely in the screw slot, and tie it in place vertically with wire so I can plug it in and let it heat for 10 or 15 minutes. Then unplug it, let it cool, and apply more penetrating oil. This alternating heating and cooling seems to help draw penetrating oil deep into the threads because of the repeated expansion and contraction. And it may be necessary to do it once or twice a day for several days or even weeks. Patience very important when removing stuck screws. The drill press method is excellent, but the ability to apply greater force at a strong downward angle also means that you may break a driver bit and do damage, or you may even strip or gall the threads if they have not had enough time for a penetrating lubricant to get way down deep to juice them up.
Remember that rust occupies much more volume than clean uncorroded steel, and it makes an abrasive thread-locker that can cause galling if you apply too much force too soon. This is why tapping helps. It compresses and breaks that bond, and also allows penetrant to get into the threads. I've had screws that I thought were hopelessly stuck and would need drilled out easily break free after several months of soaking. It also helps to cover the area with Saran Wrap to keep the penetrating oil from evaporating for these extended soaks. This is why it helps to have other projects to move on to while waiting.
