If you want to sand and polish a room next to a already sanded area. The best place to join the two areas is in a doorway. You should go lengthwise with the boards, otherwise you will se a cross line where the new coating meets the old. I have done it in the middle of rooms aswell, but the shorter the join the better. I have done half a house a lot of times for customer who don't want to store their furnitures in their garage or elsewere.