First of all the lighter the better on garage doors.
A palm sander or oscillating sander would be the best to strip. Start off using lower grit sand paper, 80. Then go up in grit to 220 or so. Any cracks or blemishes can be filled with a wood filler and sanded down with 220. Usually the prep work is the hardest and the most time consuming. It is also the most important. This would be a good time to change out the weather stripping on the bottom and around the sides. Use galvanized nails and the trim around the door will be a pvc material. On the bottom seal use large headed nails. You need to take these off to get the best results already. You can roll the flat surfaces and brush the uneven surfaces. Use a good quality latex based outdoor paint. Apply 2-3 coats and let dry. After 4-6 hours depending on the humidity install the seals. The bottom seal has a bigger side and this should face inward. Nail this on with the nails every 2-3 inches Install the top then side seals nailing them every 8-12 inches. Some are already stamped with holes.