Lay a tarp down on a workbench in a well-lighted area or take the tarp and painting outside on a sunny day. The process can be slightly messy, so cleaning the painting outside is a good choice.
Purchase two to three loafs of bread. The amount of bread depends upon the size of the watercolor painting. Some art restorers prefer sourdough bread but other types of bread work as well.
Dab or gently rub the painting with the pieces of bread. The soft, doughy texture of the bread picks up the dust and dirt on the painting without harming the watercolor. Wipe the painting with a piece of bread. As the bread becomes dirty, replace it with a fresh piece. Continue until the watercolor is completely cleaned.
Brush the painting with a soft-bristle brush to remove the excess bread crumbs. Examine the painting to make sure all the dirt and debris has been removed.