A vendor of a graffiti-resistant clear topcoat we use has put together this quick video about graffiti removal. I always recommend starting with the mildest cleaner you can use. Harsh solvents can damage the topcoat and make it less effective later. Rubbing alcohol has been effective, and it’s the solvent I recommend starting with. Also, always test in an inconspicuous area and observe the results before going ahead with full cleaning.