I haven't used Restorz-It but have used similar products in the past. This type of wood refinishing product could work for your door.
They do have some limitations. Generally major damage to a finish will require some sanding, restaining and the application of a finish (varnish). Major damage is a flaking or cracked wood finish. If you door isn't showing any of these signs, then this product is worth trying as long as your doors color isn?t too dark.
If Restorz-It doesn?t work try the following.
1) Lightly sand the door with 150 grit sand paper or a fine sanding sponge. You don?t have to strip the old finish, just clean it up and produce a smooth dull surface.
2) Find a matching wiping stain and apply it with a clean white rag to the entire door surface. Pay special attention to the faded bottom of the door and apply a little less to the top half. In other word, blend the bottom with the top using the wiping stain.
3) Allow the stain to dry for 24 hours, or over night, and then apply a good spar varnish.
I would give Restorz-It a try before spending the time with a quick "refinish".