Any scratches or marks will show when you put your transfer down, which will give it an aged look if you sand it after it dries. If you want a less aged look, make sure your plaster is completely smooth before you apply your transfer. My favorite transfer is a Crystal Clear transfer, but you can also use waterslide decals, Citra Solv transfers, blender pens or wet gel transfers. For more on image transfers, check out my book Image Transfer Workshop.
Sanded waterslide decal with paint applied to top.