Try a battery impact driver ,usually works.

Sellys EZY GLIDE - comes in a spray can. It worked well for a really salted zippper. It smells like WD-40, it is just more expensive - probably the same stuff ?

When it comes to undoing stubborn screws and bolts , I have discovered with years of experience that it helps to attempt to turn the screw not only in the direction required to loosen it but also in the direction to tighten it, only slightly though, you only want to "crack" the hold on the screw. Using brute force in the direction of loosening invariably results in the head of the screw being sheared off. May not sound effective to some but I can assure you , it does work. J

All of the above are worth a try.

Did you get new screws with smaller fins? (Stainless I hope)

Last thing you want to do is damage your board so it maybe be better (if possible ) to cut/break old fins to allow the screws to fall out.

Then think about a silcone spray (or similar) when reinstalling new screws, do not over tighten, and every now and then crack the screws so same won't happen again.

Think this is what Jack means, they do work, just support board very well.

http://www.ehow.com/how_4809810_use-hand-impact-driver.html