sanding/staining front door question

I have an oak front door that was previously stained and polyurethaned sort of a dark walnut color. Since the door faces west, the sun beats on most of the day and the polyurethane has worn off , but the stain for the most part is still there. I want to restain it the same color. Is it necessary to sand down to bare wood, or can I just sand it to the point to take a coat of new stain of the same color? I know that after the stain, I have to lightly sand with 220 grit sandpaper before I put the finish coat of polyurethane.
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Mikepier wrote:

it wont stain evenly if some parts have poly on them and some parts do not. Its the front door, so don't skimp.
What kind of wood is it?
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dnoyeB wrote:

It's an oak door 6 panel. Ok, my next question is what's better to sand, an orbital sander or the chemical stripper you brush on?
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...

Sanding down to bare wood IS the point where it takes the new stain.

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