Assuming the car is gel-coated, Meguiars has a line of compounds, polishes and waxes for boats. Start here and click around their site for more info:
I know some folks that have had sucess with Gel Gloss, a product intended for fiberglass shower and tub surrounds. You can find it at any home goods stores, like Home Depot or Lowes.
Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
2005 Intermeccanica Roadster
If you own a TDr and are not in the Registry, please go to https://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)