Now,I'm lucky enough to be married to the daughter of an ex professional sprayer,not saying he did it for me , but gave me all his tips.basically rough sand all the plastics, then spray plastic filler primer , sand flat and repeat in thin layers,when you think it's smooth spray a dark Matt colour on and sand it all off again and you will see where you have missed any low spots,then your basically ready for paint.It is a lot of work,and it has to be a flexible plastic filler primer. Sorry if that waffled on a bit...