It’s best to use the more rigid black pots for this or double up pots with flexible sides so that they don’t collapse when the planter is heavy with water. A lightweight potting soil will also help. Another bonus of lifting the planter higher is you don’t have to bend to water it. Overturning a plastic container in the bottom of the larger pot will cut down on the amount of soil needed and the weight. If there is room around the side, you can fill them in with plenty of bunched up newspaper. The newspaper helps to hold moisture and it eventually decomposes, making room for expanding roots. Don’t worry, it’s perfectly safe to recycle the newspaper in the garden. Clean the pot well and then roughen the surface with some sandpaper, so the paint will adhere better. Spray paints work much, much better than trying to brush paint on. A coat or two of primer and then two to three coats of paint, followed by a sealant should do it. You can get creative with shapes, use string or tape to create patterns (remove the string or tape once the paint dries), or use one of the metal or stone paints, as was done here. The coating won’t last forever outdoors, but you should get a few seasons of use before you need to touch them up.