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That is a controversial topic! Some people leave it bare, some varnish both sides, some varnish only the top leaving the bottom bare so it can breathe, some apply an oil finish.
I varnished both sides and it has held up well since the Spring of 2000 and the canoe has been outside since then.
and some folks use plastic cane which needs no finishing, though I really don't know how well it holds up if left out where sun and rain can get to it. It is less expensive and is worked dry.
I used the "plastic" cane from HH Perkins. When I got it, I thought they had shipped me the real cane by accident! It peels just like real cane, I had to melt it with a lighter to verify it was plastic. I cannot recommend it strongly enough. It retains it's shape even when wet, and requires no maintenance. Use the plastic, IMHO.