My future father-in-law's back door was starting to rot at the bottom and he asked me about it. He said he wanted to know if he needed a whole new door. I told him I could just patch it up with some epoxy putty and he looked at me a little funny. Once the job was done, he was pleasantly surprised at how good it looked and he was surely glad he didn't have to buy a new door.
I made a video on the method I used, maybe you could use it too!