We had a fireplace mantle made by our carpenter. He doesn't do the finish work so I am attempting. On the front corners on both sides, there is a dark line where it was connected. I don't have much experience but jump in when I can! I am posting a picture. My question is, I used wood filler to no avail. I have two layers of tung oil on and it just seems very obvious to me. Can anyone help me to make this blend with the natural maple? Thanks so much!