Paint should have a nice, clean surface. You don't want what painters call "trash" getting into your work. Sweep the surface carefully and then spray it down with a hose to get rid of any little bits of debris and dust that remain. Some homeowners find success in using a push broom to thoroughly wash the composite decking with soap and water.
The cleaner you can get the surface, the better it will accept the paint you're getting ready to put on. Let the surface dry completely and you'll be ready to move forward with the job. You're taking a big risk if you paint without putting primer on first. Some composite decking is ready to accept paint without primer, but you'll probably do better to prime anyway. The primer will give you the smoothest surface possible.
You can use either a brush or a spray gun to apply the primer. Once it has dried, you can use an approved exterior paint to get the color you desire. You will get better results with two separate thin coats than a single thick one. Once it has dried, you can replace your furniture and plants.
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