Obvioulsy you'll lose the grain if you sand them. Which may cause unaesthetic areas of grained and ungrained plastic next to each other. Depending on the material - it may be TPO (the back of the panel will tell you), in which case, getting paint to stick to it AND stay on is VERY difficult. Try a salvage yard - most Xfires have front end damage, so the trunk may be OK. If it's not that badly scuffed, leave it.