I dont know what primer to use on my bobtail spoiler. Its a genuine rubber one with ford stamped on the back. I have both plastic primer here and the etch primer i have been using for the rest of the car, can i use either of these?

I believe plastic primer would be the best for this.

is that speaking from experience? I just dont want to stuff this spoiler cos they arent chea to replace unless i get an aftermarket fiberglass one

Have a look in the technical documents section. There is a write up in there on painting plastics. It sugests to use plastic primer on the rubber quarter window moulds found on EF/ El falcons. The best thing to do would be to phone a panel shop and ask for advice.