Before you touch the battens with a sander - rig your sail with the battens just tight enought to take the wrinkles out. Rig the boat, raise the sail, set basic settings (downhaul to take out wrinkles, rotator pointed around shroud, not oversheeted), put the boat on its side (support the mast a short ladder near the hounds), and measure the draft and its position on the sail. Then you will know where and how much to sand, if you need to at all.

Sanding before measuring might get you the need to buy new battens.

On my 18, the older sail I had required all kinds of different tensions on different battens to get a consistent shape. My new sail only required batten tension to take out wrinkles, and had a wonderful shape without having to do any shaping of battens.

Measure first! Sand later!