optimum,have the door taken off bring it to a stripping place who will immerse it in chems,leaving it bare but likely with chinks & chunks & gouges,
after stripping back at the house,lay it on a sturdy table,use a belt sander to smooth out every surface,a rechargeable B & D hand sander with a triangle
buzzing scraper on the end,this gets the grooves & decorative chiseled wood art work all clean,,fill holes & gouges with powered wood putty,
( we assume you are going to paint it)
smooth out the putty spots with grain of sand paper to match the surfaces
as we have an outside door,use oil primer,on it, getting all edges make sure the door fits snug at hanging,reinstall door hardware too,mount in the frame,knock the hinges into place before repainting,let the door dry & set for a couple days,then the top coat.
a measure short of all this would be to use a heat scraper to peel off the high spots,sand the bumps,prime & paint with the door in place.