Removing paint from a pretreated wood deck will be easier than removing it from an unfinished deck. If the wood has been pressure-treated, stained or waterproofed it will be less absorbent so the paint won't penetrate the wood fibres as deeply. However, it can still be a tedious job to remove or strip paint. You have several options for removing it, but expect to spend some time on a messy job, no matter what option you choose.
Power-wash the deck, using a power washer that delivers at least 2000 psi (pounds per square inch) of pressure. This will remove all of the loose or peeling paint. You can scrape it off by hand using a chisel-edged paint scraper, but it will take you much longer.
Allow the deck to dry for at least 24 hours, then go over it again with the scraper. When wood is wet, it swells up, even if it's been pretreated. After it's dried, you'll notice that more paint has been "pushed up" and will be loose around the edges.
Apply paint stripper to the remaining paint. Follow the directions for the particular stripper---there are many different formulas. For most, you'll need to brush the stripper on, let it work for 20 to 30 minutes, and scrape or hose it off. Most paint strippers are less effective if used in full sun, so work in the shade if possible. This may take more than one application, especially if there are several layers of paint.
Hammer or screw down all the loose fasteners on the deck. Running a wide-blade scraper over the boards will alert you to proud (sticking up) nails or screws as the blade will hit them. If you don't do this, you can tear expensive sheets of sandpaper in a second.
Sand the deck with a belt or palm sender to remove the last shreds of paint. Avoid using a rotary sander if you're going to stain the deck because it will sand the wood across the grain, leaving scratch marks that will show up after staining. Use 50- or 80-grit sandpaper.
Use a wire brush to reach the gaps between boards when using paint stripper. If the task seems impossible, pry up the decking and reinstall the boards with the painted side underneath.
Be careful when using a power washer. Wood is softer than paint, and you can easily gouge the wood when removing paint.