Preparing before painting

1. Unwrap the model using Mark Seam or with Lightmap Pack.

2. Make some space between UVs using Individual Origins, and resize with "S".
3. Create a new image with transparency.
4. Paint over all the object in 3d view with a blank color to fill all the UVs inside.
5. Begin to paint your desired painting.

Please watch the video presentation

The drawing need to pass the UVs borders

For Artistic Painter to work adequately is necessary that your drawing to pass the UVs borders.

(as in the reference)

This thing can be realized in Blender, but you can do it as well in Photoshop.

Click here for Photoshop video

For Gimp is the same technique, but you need to have installed Heal Selection/Transparency plugin.

If you don't have Photoshop, use UV/Image Editor from Blender and draw more passing the UVs borders, or if you unwraped with Lightmap Pack you can easier redimension the pieces with Individual Origins and leave a bit of drawing outside the borders.

Baking for export 

Watch the video baking here