Your grp has a plastic coating which is what you want to polish. There are a number of compounds and techniques to bring back the new polished look. Basically, you are going to remove the top layer of plastic. This must be done carefully or one of two things will happen. You will wear through the top plastic coating and/or you will leave visible scratches. Every time you do this the top shiny plastic coat gets thinner. Others have mentioned compounds and ways of polishing which are all valid. If using an electric polisher, be careful that the area being polished doesn't melt with friction heat. Keep the polishing mop moving! I would like you to understand the process so that you can make good decisions. You need to remove a VERY thin layer off of the top layer of plastic. To do this you can use a multitude of compounds but they all have one thing in common; they are abrasive. You can use "wet or dry" paper, followed by a rubbing compound. I use 3000 grit wet/dry. Use it wet. Be gentle in circular motions. Dry off regularly to see how it's going. You are looking for a flat slightly matt finish. Then I finish with "T-cut" car paint restorer which brings up a stunning shine. When using rubbing compounds, use with a cotton cloth. You can buy special rubbing cloths as used in the car industry. Having used them, they are good. When you are removing the top layer of plastic with rubbing compound, you will feel a slight resistance. If no resistance, it is not working! One reason for this is too much rubbing compound, or too little, on the cloth. Polishing grp is the same as polishing car body paint and the same methods can be applied. I hope this helps.