Using an electric sander makes the job easier. A handheld orbital or belt sander is fine for small decks, but for larger ones, consider renting an upright sanding machine.
The sandpaper you need for a deck should be between 60 and 80-grit, while 100 is the maximum for the upright sander. If your deck has railings, use an 80-grit sanding sponge.
Wear a safety mask and goggles so you’re not inhaling fine wood and paint particles. Sand with the orbital sander, and go with the grain in the direction of your wooden slats.
Keep even pressure on the machine — you’re removing old stain or gloss, evening out the wood, and creating a slightly rough texture that will allow the stain to penetrate.
To clean up the sanding dust, run the vacuum cleaner over the deck before gently washing it again. Apply your choice of finishing product as per the manufacturer’s instructions.