My 1986 GN is in pretty good shape paint wise except for the urethane fill behind the front bumper and just ahead of the rear bumper. All 4 of these are grey and cloudy looking. I have waxed them and made them shiny only to have them grey and cloudy by the next day. I have also tried cleaning and removing all wax, grease, dirt, etc. and re-waxing but still by the next day it is back to looking terrible. What would cause this?