How thick is it normal to make GRP for car bodywork. I have seen 600 gsm and 1800 gsm (5 layers of 300g) quoted which is quite a wide range. Any suggestions?

Presumably won't it vary accourding to the structural loads applied through it. IE for a Locost type "skin" with little if any forces applied to it 600gsm would be thin but okay. Obviously for a monocoque you'ld need more....

600 is about the norm for locost bodywork

I've always done :-

1 x layer 600g (2 ounce)
1 x layer 450g (1.5 ounce)

on all locost type applications.