couple of things.
eel is right, BUT-- many time "100% solid wood doors" are still a solid core with your fir/oak/maple/etc... veneer over the top. this is true no matter the price tag.( you can still pay thousands on a veneer covered, solid wood door)
so be VERY carefull when sanding as to not sand through a veneer.
2nd-
SEA THIN TEAK OIL!!
it is absolulty the best product you can put on a wood door that will be out in the weather. any teak oil will be good, but sea thin teak oil is the beat.
lay the door on some saw horses, dump the oil on the door, and hand rub it in. rub in in good! then simply take a sataining sponge and wipe off the excess. let it dry, flip the door, do it again. no runs when you lay it flat, the drawback is it takes 3 days to finsh the door, but it will be done perfectly, and as high quality as you could ever pay for.