What type of door. Is it a solid wood door or the fake wood type? I you really don't want to put anything on it because anything that soaks into or stays on the door will effect the darkness of the stain. The stain will soak in less in the area where the wood has already soaked up something. I would just sand it with 220 grit sandpaper and then stain it. The sanding will also take out small scratches and nicks. If it is two badley stained you will just have to prime and paint it or replace it.