If the tree is rotten I think better not to use it for woodworks, because this will have a big impact on your final result. Recently I brought some trees from the forest that looked good and didn't look rotten and I started to cut the wood into several pieces to get some boards and they got pretty okay but they had to be sanded quite well. For the projects I had to do, I had doubts that something normal would be received,but I still tried. I thought it would be easier for me to sand when I will try to use an orbital sander
, because it have a better result on woodworks and big surfaces and to my surprise it actually worked. In your case, you can try but again this is not 100% effective.