mahogany door refinish

My builder sent out a slip that said the put bullseye shellac on my mahogany front door every year to keep it looking its best.
Shellac is a pretty weak finish for use. I'm was thinking more like Helmsman spar polyurethane to hold up to the elements.
The top and bottom of the door are not finished. Should I seal those or leave them open to let the wood "breath".?
What would you guys recommend for finishing a mahogany exterior door.
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Use a Marine product it will flex better, Helmsman is ok.
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mook Johnson wrote:

Seal.
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dadiOH
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