What can I use to clean the outside of my terracotta pots for the Flower show?
It is difficult to clean a terracotta pot with the plant in the pot. The first step is to use a spray nozzle that most kitchen sinks have attached. Set the water at room temperature by adjusting the cold and hot water faucets. Place the pot in the sink and spray lightly around the pot. It is usually best not to get water on the plant. Brush the pot with a brush having medium hard bristles. It's best to work around the pot turning it every so often. Clean the pot rim without disturbing the soil. Also, take a light pass at the bottom of the pot by lifting it up a few inches and scrubbing the bottom. Rinse off the pot and examine it for dirt spots and stains. Most of the loose dirt should have been removed. If necessary brush the pot again. A paste made up of baking soda and applied to the pot may be a good alternate first step, brushing as necessary, then rinsing.
Some remaining stains will not show up until the pot is dry. Sand remaining spains using a medium or fine sand paper. The foam bricks covered with sand paper are ideal for this. The pot can be dry or wet, your choice. I have found that wet works well, but the sand paper loses it effectiveness when wet. When satsified that the spots are removed, rinse with room temperature water.
When dry if the pot still shows stains. they can be temporarily covered with a light vegetable oil. Just put oil on a folded paper towel or sponge and wet the entire outside surface of the pot. Dry with a paper towel.
There are commercial sprays in an aerosol can that can be applied. There is a strong chemical smell when you use them. I would be reluctant to use them on pots with plants in them. Similarily, bleach solutions are used to clean and sterilize pots, but not with plants in them.