Self build Conservatory questions....

I'm building a conservatory out of standard unsorted joinery.
1. I have a long piece of 7x3 which has some shakes I cannot get rid of completely. Is there a good way to seal up the shakes before painting? I'm assuming that it'll involve stuffing in some glue or something like that. Any tips appreciated.
2. I want to incorporate roof vents but don't like the look of the horrible lead cowl on the ones I've seen so far. Does anyone have any ideas as to a better design?

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