I'm not sure why you have ripples, but I used to vac form with polystyrene. Can you take a picture with better lighting and focus on the texture itself? If it is just a low profile bit of roughness you can use sandpaper with increasing grits until you like how smooth it is. If it has little holes and bumps and pocks that are noticeable, you may need a combo of sanding, filling with epoxy, and painting to level things smooth.