Corel Painter Help :
Shapes : Converting shapes to image layers
Quick links to procedures on this page:
In Corel Painter, the shapes that you create are added to shape layers. If you want to alter a shape by painting, applying effects, or transforming, you must convert the shape to a pixel-based image layer. You can convert an individual shape or a group of shapes to an image layer.
Shape layers vs. image layers
Some of the options and controls for working with pixel-based image layers also apply to shapes. For example, you can move shapes in the same way you move image layers, and you can change the composite method to control how the shape interacts with the underlying image. Shapes also follow the same layering rules as pixel-based image layers, and you can manipulate them in many of the same ways. However, shapes differ from pixel-based image layers by the type of data they contain. Shapes are vector objects; pixel-based image layers are constructed of pixels. If you want to apply brushstrokes, transformations (using the Transform tool), or effects to a shape, you must convert the shape to a pixel-based layer. Moreover, if you attempt to perform any of these actions with a shape selected, Corel Painter automatically displays a warning message to instruct you to commit the shape to an image layer. After you commit the shape to an image layer, you cannot re-access the shape-specific controls.
For more information, Layers.
You cannot paint on a shape using Watercolor brushes or Liquid Ink brushes, because they automatically create their own special layers.
In some cases, Corel Painter will prompt you to convert a shape. For example, if you attempt to apply an effect to a shape, Corel Painter asks if you want to commit the shape to an image layer.
You can also convert a shape to an image layers by choosing Shapes 
Convert To Layer.
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, and choose Convert To Default Layer.
in the Layers panel.