Thanks for all the ideas!! I was looking for a silver bullet, but there ain't none. Chris, I tried heat, but it didn't help. Likely operator error-I am good at that. One rail has been removed by turning the canoe on its side on horses: cleaning the slots as well as possible, inserting the slotted screwdriver ground to the correct size, tapping the screwdriver with a hammer, turning clockwise, and then counterclockwise. with all the weight of my 145lb body could apply. Only one would not respond properly. A Dremel deepened that slot enough so it could be removed.
I don' recall (I used white lead filler for years so my brain could be fried} ever having this much trouble removing outwale screws. I thought of a brace and bit , but mine has right handed threads so it would simply loosen the bit. That is after likely 500 restorations( I never kept track, but it has been since "87 )
This is a very special canoe that has spent its entire life on Devils Lake. Michigan, and even though the family does not know it is very likely to spend the rest of its life at the same place, at the same cottage( every lake domicile is a cottage ,no matter the size).
Thanks again,
Gil,