My front door is stained and urethaned the stain seems to be still in good shape, mostly peeling on bottom of door where water hits sometimes. Can I just put urethane on bottom of door and blend it?
Yes you can, but you probably will not be able to blend to your satisfaction. I have been re-habing wooden front doors for 30 years and have not been able to blend to a satisfactory end. Stripping is the best option in my opinion.
If you wish, sand and varnish with 3 or more coats. I will usually add a stain to the first coat. Find one close to the existing door color. Be sure to stay in the grain pattern if you do this and allow to dry over night before the second coat. Using a semi-gloss varnish has produced the best results for us. I prefer Pittsburg Paints or Minwax Semi-gloss Polyurethane. You will need to schedule an annual coat or two to prevent the damage re-occuring.