If you have teak trim on your boat, then at some point you will be sanding it to maintain it. Whether it is by hand or with power tools, you have to be careful of the surfaces around the teak. I put two or three layers of painters tape down on the perimeter of the wood to protect adjacent surfaces from the inevitable accidental slip of the sandpaper.
It protects the paint or metal and gives you a indicator that you are touching where you don’t want to. The multiple layers of tape give you some forgiveness if you hit it with the sandpaper. It will eventually go through the tape too, so you still have to be careful. I use an orbital for the open flat surfaces. You almost have to do the edges by hand. Always sand with the grain!