Painting landscape materials is great (https://docs.unrealengine.com/latest/INT/Engine/Landscape/QuickStart/5/index.html).
But once I export the landscape material into a new terrain, the painted materials just disappear. How to keep these grass, snow, soil materials?
Thank you in advance.
You need to associate layer info to each layer before painting on the terrain. Layer info is something like a save file for your paint data.
As stated on the docs link you provided:
You can, however create a “smarter” landscape material that, for example, applies a rock texture wherever is a slope. Here is a playlist on youtube to get you started: Landscape Height, Slope blend with the ability to paint on layers
