Refinish Old Oak Exterior Doors

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I am refinishing our 100+ year old exterior oak doors. They are in pretty good shape as the finish has eroded (gray wood exposed) in only a few spots. I think this is due to the fact that the doors face north and thus have no exposure to sunlight and they were probably redone regularly. I don’t want to strip the finish since they are in good shape, but rather sand and apply marine varnish w/o staining first. How much sanding needs to be done on the old finish? Would scuffing them up thoroughly with 150 sandpaper do the trick? On the few exposed (grey wood) areas should I sand onlly minimally with maybe 180 since there’s no finsh on them? Should I apply something (sanding sealer) to the bare grey wood areas before applying finish? What grade sandpaper should I use before applying finish? What grade sandpaper should I apply in between coats? FEEDBACK APPREICATED – THANKS!