The best sandpaper to use would be a 320 grit. You will need to take caution to follow the grain of the wood when sanding, never sand against the grain as it could cause damage to the wood. After sanding your teak chair, you will want to lightly dust the chair with a soft cloth to remove any residue left over. Your final step will be to lightly coat the chair with teak oil to preserve the luxurious look.
These seats are manufactured specifically for the outdoors. Dragging your indoor chairs outside will do you no good. Using these pieces is way better for they can endure long exposures to harsh weather conditions.