I had the same issue during refurbishment of my White FR ; I had tried everything mentioned on this board; what worked best, simplest, least messy, and least smelly was a few weeks of repeated applications of Kroil Oil and Mean Green spray.
I had to sit the toughest stuck parts in a shallow bath or Kroil Oil which creeps into the smallest of air pockets to moisten the dried grime that is the cause of the issue; then I sprayed with Mean Green and scrubbed with an old tooth brush and paper towels to dissolve away the loosened grime to reveal shining metal and released screws. I had also torqued the screws between each bath with a leather scrap wrapped around the head and secured with locking pliers. And for areas that I couldn't logistically place in a bath bowl I had to wet a paper towel in the bath and wrap the stuck area with the soaked paper towel.
You can get the Mean Green spray at WalMart; and I got my Kroil Oil from Kano Labs. Also the technical term in the correct screw drivers mentioned by Mickey2 is "hollow point tips", visually the tips are tapered. I don't know the science behind why these things all work but in my laymen experience, they do. Self patience and diligence are helpful too.
Best of luck with it.