Staining over existing deck paint is fairly easy. You do need to be aware that your stain will not necessarily look much like it does on the can if you apply it directly over the paint. However, it will perform the duties that you generally want stain to do, such as preserving the wood and helping to turn away the worst of the elements. If you want to stain over existing deck paint, be sure to test the results in a small area first. This will help you make sure that you will love your finished look before it is too late to make a change in your plans.
Give your deck a thorough scrubbing. Use 2 cups of white vinegar and 2/3 of the bucket of hot water to make a scrubbing solution. Scrub the deck with this solution to remove dirt, mold, mildew and any loose paint chips. You can scrub using the bristle broom.
Staining over paint is hard because paint does not accept stain the same way that wood will. It may look good temporarily, but it will not last nearly as long as either media on its own. To get the longest life out of your stain-over-paint look, you need to get the painted surface as clean as you possibly can.
Rinse off the deck. Use the hose to rinse the deck thoroughly. If you notice any remaining grime, go back and scrub it off before re-rinsing. Let the deck dry completely before moving on.
Sand the deck very lightly. If you are staining over paint, then clearly you do not wish to remove the paint. However, the stain will have trouble sticking to a smooth, painted surface, so you need to rough it up a little. Use light pressure and sand only with the grain of the wood so that the stain will pick up the grain once you apply it.
Sweep off the deck. This way you will not have dust from the sanding stuck in your stain.
Start staining. Simply use the brush to apply a relatively thin, even coat. Keep an eye on the way that the stain is reacting to the paint. You can use a rag to smooth the stain into the deck more effectively if it is forming droplets and refusing to adhere to the painted surface. If the rag wipes the stain off the paint cleanly, then you may have to sand more thoroughly in order to get the stain to stick.
Let the deck dry. It is very important that no one steps on the stained areas of the deck while they are drying. Keep everyone off your deck, and the deck dry, for at least 48 hours. If it is humid, you may need even more time.