ArtRage currently doesn't have a way to edit the default canvas. When starting a new painting, you can use the "Cel" preset if you want a completely smooth transparent paper, or if you want a smooth opaque paper, use the Cel preset as a basis and turn the opacity to 100%, then saving that preset under a different name for future use.
If you've imported an image to start a new painting, the quickest way to change the paper texture is to right click on the layer panel, select paper settings and click the preset you want to use.
Beyond that, if you've started an image, you shouldn't need to apply any paper settings changes beyond the base layer, as all subsequent layers adopt the same canvas texture by default, so if your first layer is set to 0% roughness before you add further layers, all further layers you add will be at 0% roughness, likewise if you've imported an image to start apainting, then set the paper to 0% roughness, all further layers will adopt that setting by default. ( New layers adopt the paper settings of the layer beneath them ). Hope that's of some use.