Hi little jack
Mer is a polish and although I used it for years there is far better on the market.Mer will only make a vehicle shine if it does not need a mop or cut.
Any automotive colour restorer such as T-cut or Farecla G3 wil remove the oxidization from GRP & using car wax will keep it shiny for a while, it works on my Eriba caravan roof & also on my camper roof.
Presumabley gel coat restorer would be better. Farecla also supply mops if you want to do the job mechanically.
wynthesurfer, I am talking about restoring the shine to GRP and not what I thoink, I`ve been in the motor body trade for 50 years repairing and restoring GRP bodied cars