Sand the unit smooth to the touch with 220-320 grit sandpaper. Even if its smooth quick sand it. From there you will want to etch and deep clean with Lysol Power or any toilet bowl cleaner that has hydrochloric acid in it. This will etch and deep clean the surface. You can fill any cracks, chips, holes, dents, or pitted areas with automotive bondo and then sand smooth for a more level finish. Then you can tape up and start to pour.